mach-tui 0.3.0

//! SQLite-backed persistence for tasks, categories, and settings.
//!
//! A [`Store`] is an explicit instance: callers can open independent data
//! directories in one process. Writes use `BEGIN IMMEDIATE`, validate a fresh
//! snapshot, and commit tasks/categories/settings/attachment metadata plus a
//! monotonic revision in one transaction. Image bytes are immutable,
//! content-addressed files beside the database. Legacy JSON is imported once
//! and left untouched.

use std::collections::{HashMap, HashSet};
use std::fs;
use std::io::{Read, Write};
use std::path::{Path, PathBuf};
use std::time::Duration;

use chrono::{Local, NaiveDateTime};
use image::ImageFormat;
use rusqlite::limits::Limit;
use rusqlite::{Connection, OptionalExtension, Transaction, TransactionBehavior, params};
use serde::Deserialize;
use serde::de::DeserializeOwned;
use sha2::{Digest, Sha256};
use unicode_normalization::UnicodeNormalization;
use unicode_segmentation::UnicodeSegmentation;

use crate::due;
use crate::model::{
    Block, Category, MAX_BODY_LINES, MAX_CATEGORY_COUNT, MAX_CATEGORY_DESC_LINE_LEN,
    MAX_CATEGORY_DESC_LINES, MAX_CATEGORY_NAME_LEN, MAX_IMPORTANCE, MAX_NOTES_LINE_LEN,
    MAX_TASK_COUNT, MAX_TITLE_LEN, SCHEMA_VERSION, Task, caseless_key, text_byte_limit,
};
use crate::settings::{DATE_FORMATS, PREVIEW_POSITIONS, SORTS, Settings, THEMES};

const DATABASE_FILE: &str = "mach.db";
const DATABASE_SCHEMA_VERSION: i64 = 1;
const LEGACY_MIGRATION_KEY: &str = "legacy_json_migrated";
const BUSY_TIMEOUT: Duration = Duration::from_secs(10);
const ID_MAX_BYTES: usize = 128;
const DUE_MAX_BYTES: usize = 128;
const CREATED_MAX_BYTES: usize = 64;
const SETTINGS_VALUE_MAX_BYTES: usize = 128;
const MAX_LEGACY_JSON_BYTES: u64 = 128 * 1024 * 1024;
const MAX_SQLITE_VALUE_BYTES: i32 = 8 * 1024 * 1024;
const MAX_ATTACHMENT_BYTES: u64 = 128 * 1024 * 1024;
const ATTACHMENT_ID_LEN: usize = 64;

#[derive(Debug)]
pub enum StoreError {
    Io {
        operation: &'static str,
        path: PathBuf,
        source: std::io::Error,
    },
    Json {
        path: PathBuf,
        source: serde_json::Error,
    },
    Database(rusqlite::Error),
    UnsupportedLegacySchema {
        path: PathBuf,
        found: u32,
        expected: u32,
    },
    UnsupportedDatabaseSchema {
        path: PathBuf,
        found: i64,
        expected: i64,
    },
    Conflict {
        expected: u64,
        actual: u64,
    },
    StaleEntity {
        entity: &'static str,
        id: String,
    },
    NotFound {
        entity: &'static str,
        query: String,
    },
    Ambiguous {
        entity: &'static str,
        query: String,
        matches: Vec<String>,
    },
    Validation(String),
    Corrupt(String),
}

impl StoreError {
    pub fn validation(message: impl Into<String>) -> Self {
        Self::Validation(message.into())
    }

    fn io(operation: &'static str, path: &Path, source: std::io::Error) -> Self {
        Self::Io {
            operation,
            path: path.to_path_buf(),
            source,
        }
    }
}

impl std::fmt::Display for StoreError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Io {
                operation,
                path,
                source,
            } => write!(f, "could not {operation} {}: {source}", path.display()),
            Self::Json { path, source } => {
                write!(f, "could not parse {}: {source}", path.display())
            }
            Self::Database(source) => write!(f, "database error: {source}"),
            Self::UnsupportedLegacySchema {
                path,
                found,
                expected,
            } => write!(
                f,
                "{} uses unsupported schema {found} (expected {expected})",
                path.display()
            ),
            Self::UnsupportedDatabaseSchema {
                path,
                found,
                expected,
            } => write!(
                f,
                "{} uses unsupported database schema {found} (expected {expected})",
                path.display()
            ),
            Self::Conflict { expected, actual } => write!(
                f,
                "store changed since it was loaded (expected revision {expected}, found {actual})"
            ),
            Self::StaleEntity { entity, id } => {
                write!(f, "{entity} {id:?} changed since it was loaded")
            }
            Self::NotFound { entity, query } => {
                write!(f, "no {entity} matching {query:?}")
            }
            Self::Ambiguous {
                entity,
                query,
                matches,
            } => write!(
                f,
                "ambiguous {entity} {query:?}; matches: {}",
                matches.join(", ")
            ),
            Self::Validation(message) => write!(f, "invalid data: {message}"),
            Self::Corrupt(message) => write!(f, "corrupt database: {message}"),
        }
    }
}

impl std::error::Error for StoreError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::Io { source, .. } => Some(source),
            Self::Json { source, .. } => Some(source),
            Self::Database(source) => Some(source),
            _ => None,
        }
    }
}

impl From<rusqlite::Error> for StoreError {
    fn from(value: rusqlite::Error) -> Self {
        Self::Database(value)
    }
}

#[derive(Debug, Clone, Default)]
pub struct StoreData {
    pub revision: u64,
    pub categories: Vec<Category>,
    pub tasks: Vec<Task>,
    pub settings: Settings,
    pub(crate) attachments: Vec<Attachment>,
}

/// Immutable metadata for one content-addressed image owned by this store.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Attachment {
    pub id: String,
    pub sha256: String,
    pub media_type: String,
    pub byte_len: u64,
    pub storage_name: String,
}

#[derive(Debug, Clone, Default)]
pub struct TaskPatch {
    pub title: Option<String>,
    pub body: Option<Vec<Block>>,
    pub due: Option<String>,
    pub done: Option<bool>,
    pub importance: Option<u8>,
    /// `None` leaves the category unchanged; `Some(None)` clears it.
    pub category_id: Option<Option<String>>,
}

#[derive(Debug, Clone, Default)]
pub struct CategoryPatch {
    pub name: Option<String>,
    pub description: Option<String>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum PurgeScope {
    All,
    Category(String),
    Uncategorized,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum RelativePosition {
    Before,
    After,
}

impl StoreData {
    pub fn attachments(&self) -> &[Attachment] {
        &self.attachments
    }

    /// Resolve a task by full id or unique id prefix.
    pub fn resolve_task_id(&self, query: &str) -> Result<String, StoreError> {
        let query = query.trim();
        if query.is_empty() {
            return Err(StoreError::validation("task id cannot be empty"));
        }
        validate_byte_limit(query, ID_MAX_BYTES, "task id query")?;
        if let Some(task) = self.tasks.iter().find(|task| task.id == query) {
            return Ok(task.id.clone());
        }
        let matches: Vec<_> = self
            .tasks
            .iter()
            .filter(|task| task.id.starts_with(query))
            .map(|task| task.id.clone())
            .collect();
        match matches.as_slice() {
            [id] => Ok(id.clone()),
            [] => Err(StoreError::NotFound {
                entity: "task",
                query: query.to_string(),
            }),
            _ => Err(StoreError::Ambiguous {
                entity: "task id",
                query: query.to_string(),
                matches,
            }),
        }
    }

    /// Resolve a category by id, Unicode-caseless name, or unique name prefix.
    pub fn resolve_category_id(&self, query: &str) -> Result<String, StoreError> {
        let query = query.trim();
        if query.is_empty() {
            return Err(StoreError::validation("category name cannot be empty"));
        }
        if let Some(category) = self.categories.iter().find(|category| category.id == query) {
            return Ok(category.id.clone());
        }
        validate_byte_limit(
            query,
            text_byte_limit(MAX_CATEGORY_NAME_LEN),
            "category query",
        )?;
        let folded = category_name_key(query);
        if let Some(category) = self
            .categories
            .iter()
            .find(|category| category_name_key(&category.name) == folded)
        {
            return Ok(category.id.clone());
        }
        let matches: Vec<_> = self
            .categories
            .iter()
            .filter(|category| category_name_has_prefix(&category.name, &folded))
            .collect();
        match matches.as_slice() {
            [category] => Ok(category.id.clone()),
            [] => Err(StoreError::NotFound {
                entity: "category",
                query: query.to_string(),
            }),
            _ => Err(StoreError::Ambiguous {
                entity: "category",
                query: query.to_string(),
                matches: matches
                    .into_iter()
                    .map(|category| category.name.clone())
                    .collect(),
            }),
        }
    }

    pub fn task(&self, id: &str) -> Result<&Task, StoreError> {
        self.tasks
            .iter()
            .find(|task| task.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "task",
                query: id.to_string(),
            })
    }

    pub fn category(&self, id: &str) -> Result<&Category, StoreError> {
        self.categories
            .iter()
            .find(|category| category.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "category",
                query: id.to_string(),
            })
    }

    pub fn create_task(
        &mut self,
        title: impl Into<String>,
        body: Vec<Block>,
        due: impl Into<String>,
        importance: u8,
        category_id: Option<String>,
    ) -> Result<Task, StoreError> {
        if importance > MAX_IMPORTANCE {
            return Err(StoreError::validation(format!(
                "importance must be 0-{MAX_IMPORTANCE}"
            )));
        }
        let title = title.into();
        let due = due.into();
        let mut task = Task::new(&title, importance, category_id, &due);
        task.body = body;
        self.insert_task(task)
    }

    pub fn insert_task(&mut self, task: Task) -> Result<Task, StoreError> {
        let index = self.tasks.len();
        self.tasks.push(task);
        if let Err(error) = self.normalize_and_validate_new_write() {
            self.tasks.truncate(index);
            return Err(error);
        }
        Ok(self.tasks[index].clone())
    }

    pub fn edit_task(&mut self, id: &str, patch: TaskPatch) -> Result<Task, StoreError> {
        let index = self
            .tasks
            .iter()
            .position(|task| task.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "task",
                query: id.to_string(),
            })?;
        let before = self.tasks[index].clone();
        {
            let task = &mut self.tasks[index];
            if let Some(title) = patch.title {
                task.title = title;
            }
            if let Some(body) = patch.body {
                task.body = body;
            }
            if let Some(due) = patch.due {
                task.due = due;
            }
            if let Some(done) = patch.done {
                task.done = done;
            }
            if let Some(importance) = patch.importance {
                task.importance = importance;
            }
            if let Some(category_id) = patch.category_id {
                task.category_id = category_id;
            }
        }
        if let Err(error) = self.normalize_and_validate_new_write() {
            self.tasks[index] = before;
            return Err(error);
        }
        Ok(self.tasks[index].clone())
    }

    /// Apply only the fields represented by `patch`, but fail if one of those
    /// fields changed since `expected` was loaded. Unrelated concurrent edits
    /// (for example toggling `done` while a title form is open) are preserved.
    pub fn edit_task_if_unchanged(
        &mut self,
        expected: &Task,
        patch: TaskPatch,
    ) -> Result<Task, StoreError> {
        let current = self
            .tasks
            .iter()
            .find(|task| task.id == expected.id)
            .ok_or_else(|| StoreError::StaleEntity {
                entity: "task",
                id: expected.id.clone(),
            })?;
        let stale = field_conflicts(patch.title.as_ref(), &current.title, &expected.title)
            || field_conflicts(patch.body.as_ref(), &current.body, &expected.body)
            || field_conflicts(patch.due.as_ref(), &current.due, &expected.due)
            || field_conflicts(patch.done.as_ref(), &current.done, &expected.done)
            || field_conflicts(
                patch.importance.as_ref(),
                &current.importance,
                &expected.importance,
            )
            || field_conflicts(
                patch.category_id.as_ref(),
                &current.category_id,
                &expected.category_id,
            );
        if stale {
            return Err(StoreError::StaleEntity {
                entity: "task",
                id: expected.id.clone(),
            });
        }
        self.edit_task(&expected.id, patch)
    }

    pub fn delete_task(&mut self, id: &str) -> Result<Task, StoreError> {
        let index = self
            .tasks
            .iter()
            .position(|task| task.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "task",
                query: id.to_string(),
            })?;
        Ok(self.tasks.remove(index))
    }

    pub fn set_task_done(&mut self, id: &str, done: bool) -> Result<Task, StoreError> {
        self.edit_task(
            id,
            TaskPatch {
                done: Some(done),
                ..TaskPatch::default()
            },
        )
    }

    pub fn toggle_task_done(&mut self, id: &str) -> Result<Task, StoreError> {
        let done = !self.task(id)?.done;
        self.set_task_done(id, done)
    }

    pub fn set_task_importance(&mut self, id: &str, importance: u8) -> Result<Task, StoreError> {
        self.edit_task(
            id,
            TaskPatch {
                importance: Some(importance),
                ..TaskPatch::default()
            },
        )
    }

    pub fn set_task_category(
        &mut self,
        id: &str,
        category_id: Option<String>,
    ) -> Result<Task, StoreError> {
        self.edit_task(
            id,
            TaskPatch {
                category_id: Some(category_id),
                ..TaskPatch::default()
            },
        )
    }

    pub fn move_task(&mut self, id: &str, target: usize) -> Result<(), StoreError> {
        let index = self
            .tasks
            .iter()
            .position(|task| task.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "task",
                query: id.to_string(),
            })?;
        if target >= self.tasks.len() {
            return Err(StoreError::validation(format!(
                "task target index {target} is out of range"
            )));
        }
        if self.tasks[index].category_id != self.tasks[target].category_id {
            return Err(StoreError::validation(
                "tasks can only be reordered within the same category",
            ));
        }
        let task = self.tasks.remove(index);
        self.tasks.insert(target, task);
        Ok(())
    }

    pub fn move_task_relative(
        &mut self,
        id: &str,
        target_id: &str,
        position: RelativePosition,
    ) -> Result<Task, StoreError> {
        let id = id.to_string();
        let target_id = target_id.to_string();
        if id == target_id {
            return Err(StoreError::validation(
                "cannot move a task relative to itself",
            ));
        }
        let index = self
            .tasks
            .iter()
            .position(|task| task.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "task",
                query: id.clone(),
            })?;
        let target_index = self
            .tasks
            .iter()
            .position(|task| task.id == target_id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "task",
                query: target_id.clone(),
            })?;
        if self.tasks[index].category_id != self.tasks[target_index].category_id {
            return Err(StoreError::validation(
                "tasks can only be reordered within the same category",
            ));
        }
        let task = self.tasks.remove(index);
        let target_after_removal = self
            .tasks
            .iter()
            .position(|candidate| candidate.id == target_id)
            .ok_or_else(|| {
                StoreError::Corrupt(format!("task {target_id:?} disappeared during reorder"))
            })?;
        let insertion = match position {
            RelativePosition::Before => target_after_removal,
            RelativePosition::After => target_after_removal + 1,
        };
        self.tasks.insert(insertion, task.clone());
        Ok(task)
    }

    pub fn purge_completed(&mut self, scope: &PurgeScope) -> Result<Vec<Task>, StoreError> {
        if let PurgeScope::Category(id) = scope {
            self.category(id)?;
        }
        let mut removed = Vec::new();
        self.tasks.retain(|task| {
            let in_scope = match scope {
                PurgeScope::All => true,
                PurgeScope::Category(id) => task.category_id.as_deref() == Some(id),
                PurgeScope::Uncategorized => task.category_id.is_none(),
            };
            if task.done && in_scope {
                removed.push(task.clone());
                false
            } else {
                true
            }
        });
        Ok(removed)
    }

    /// Purge only the completed tasks captured by a confirmation prompt.
    pub fn purge_completed_ids(&mut self, ids: &[String]) -> Result<Vec<Task>, StoreError> {
        let ids: HashSet<_> = ids.iter().map(String::as_str).collect();
        let mut removed = Vec::new();
        self.tasks.retain(|task| {
            if task.done && ids.contains(task.id.as_str()) {
                removed.push(task.clone());
                false
            } else {
                true
            }
        });
        Ok(removed)
    }

    pub fn create_category(
        &mut self,
        name: impl Into<String>,
        description: impl Into<String>,
    ) -> Result<Category, StoreError> {
        let name = name.into();
        let mut category = Category::new(&name);
        category.description = description.into();
        self.insert_category(category)
    }

    pub fn insert_category(&mut self, category: Category) -> Result<Category, StoreError> {
        let index = self.categories.len();
        self.categories.push(category);
        if let Err(error) = self.normalize_and_validate_new_write() {
            self.categories.truncate(index);
            return Err(error);
        }
        Ok(self.categories[index].clone())
    }

    pub fn edit_category(
        &mut self,
        id: &str,
        patch: CategoryPatch,
    ) -> Result<Category, StoreError> {
        let index = self
            .categories
            .iter()
            .position(|category| category.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "category",
                query: id.to_string(),
            })?;
        let before = self.categories[index].clone();
        {
            let category = &mut self.categories[index];
            if let Some(name) = patch.name {
                category.name = name;
            }
            if let Some(description) = patch.description {
                category.description = description;
            }
        }
        if let Err(error) = self.normalize_and_validate_new_write() {
            self.categories[index] = before;
            return Err(error);
        }
        Ok(self.categories[index].clone())
    }

    pub fn edit_category_if_unchanged(
        &mut self,
        expected: &Category,
        patch: CategoryPatch,
    ) -> Result<Category, StoreError> {
        let current = self
            .categories
            .iter()
            .find(|category| category.id == expected.id)
            .ok_or_else(|| StoreError::StaleEntity {
                entity: "category",
                id: expected.id.clone(),
            })?;
        let stale = field_conflicts(patch.name.as_ref(), &current.name, &expected.name)
            || field_conflicts(
                patch.description.as_ref(),
                &current.description,
                &expected.description,
            );
        if stale {
            return Err(StoreError::StaleEntity {
                entity: "category",
                id: expected.id.clone(),
            });
        }
        self.edit_category(&expected.id, patch)
    }

    /// Delete a category while preserving its tasks as uncategorized.
    pub fn delete_category(&mut self, id: &str) -> Result<Category, StoreError> {
        let index = self
            .categories
            .iter()
            .position(|category| category.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "category",
                query: id.to_string(),
            })?;
        let category = self.categories.remove(index);
        for task in &mut self.tasks {
            if task.category_id.as_deref() == Some(id) {
                task.category_id = None;
            }
        }
        Ok(category)
    }

    pub fn move_category(&mut self, id: &str, target: usize) -> Result<(), StoreError> {
        let index = self
            .categories
            .iter()
            .position(|category| category.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "category",
                query: id.to_string(),
            })?;
        if target >= self.categories.len() {
            return Err(StoreError::validation(format!(
                "category target index {target} is out of range"
            )));
        }
        let category = self.categories.remove(index);
        self.categories.insert(target, category);
        Ok(())
    }

    pub fn move_category_relative(
        &mut self,
        id: &str,
        target_id: &str,
        position: RelativePosition,
    ) -> Result<Category, StoreError> {
        if id == target_id {
            return Err(StoreError::validation(
                "cannot move a category relative to itself",
            ));
        }
        let index = self
            .categories
            .iter()
            .position(|category| category.id == id)
            .ok_or_else(|| StoreError::NotFound {
                entity: "category",
                query: id.to_string(),
            })?;
        if !self
            .categories
            .iter()
            .any(|category| category.id == target_id)
        {
            return Err(StoreError::NotFound {
                entity: "category",
                query: target_id.to_string(),
            });
        }
        let category = self.categories.remove(index);
        let target_after_removal = self
            .categories
            .iter()
            .position(|candidate| candidate.id == target_id)
            .ok_or_else(|| {
                StoreError::Corrupt(format!("category {target_id:?} disappeared during reorder"))
            })?;
        let insertion = match position {
            RelativePosition::Before => target_after_removal,
            RelativePosition::After => target_after_removal + 1,
        };
        self.categories.insert(insertion, category.clone());
        Ok(category)
    }

    pub fn replace_settings(&mut self, settings: Settings) -> Result<Settings, StoreError> {
        let before = std::mem::replace(&mut self.settings, settings);
        if let Err(error) = validate_settings(&self.settings) {
            self.settings = before;
            return Err(error);
        }
        Ok(self.settings.clone())
    }

    pub fn update_settings(
        &mut self,
        operation: impl FnOnce(&mut Settings),
    ) -> Result<Settings, StoreError> {
        let before = self.settings.clone();
        operation(&mut self.settings);
        if let Err(error) = validate_settings(&self.settings) {
            self.settings = before;
            return Err(error);
        }
        Ok(self.settings.clone())
    }

    fn normalize_and_validate_new_write(&mut self) -> Result<(), StoreError> {
        normalize_and_validate(
            self,
            Local::now().naive_local(),
            DueMode::NewWrite,
            AttachmentMode::Draft,
        )
    }
}

/// A three-way field merge conflicts only when the remote and desired values
/// both diverged from the captured base in different directions.
fn field_conflicts<T: PartialEq>(desired: Option<&T>, current: &T, expected: &T) -> bool {
    desired.is_some_and(|desired| current != expected && current != desired)
}

#[derive(Debug, Clone)]
pub struct Paths {
    pub dir: PathBuf,
    pub database: PathBuf,
    pub tasks: PathBuf,
    pub categories: PathBuf,
    pub settings: PathBuf,
    pub images: PathBuf,
}

impl Paths {
    fn new(dir: PathBuf) -> Self {
        Self {
            database: dir.join(DATABASE_FILE),
            tasks: dir.join("tasks.json"),
            categories: dir.join("categories.json"),
            settings: dir.join("settings.json"),
            images: dir.join("images"),
            dir,
        }
    }
}

pub struct Store {
    connection: Connection,
    paths: Paths,
    persistent_attachments: bool,
}

impl Store {
    pub fn open(dir: impl AsRef<Path>) -> Result<Self, StoreError> {
        let paths = Paths::new(expand_user(dir.as_ref().to_path_buf())?);
        ensure_private_directory(&paths.dir)?;
        prepare_private_database_file(&paths.database)?;
        let mut connection = Connection::open(&paths.database)?;
        set_private_file(&paths.database)?;
        connection.busy_timeout(BUSY_TIMEOUT)?;
        configure_resource_limits(&connection)?;
        initialize_schema(&mut connection, &paths.database)?;
        configure_connection(&connection)?;
        quick_check(&connection)?;
        let mut store = Self {
            connection,
            paths,
            persistent_attachments: true,
        };
        store.migrate_legacy_json()?;
        Ok(store)
    }

    /// Open an ephemeral store with no filesystem persistence.
    ///
    /// `data_dir` is only the logical base for relative image references. The
    /// directory is not created or modified.
    pub fn open_in_memory_with_paths(data_dir: impl AsRef<Path>) -> Result<Self, StoreError> {
        let paths = Paths::new(expand_user(data_dir.as_ref().to_path_buf())?);
        let mut connection = Connection::open_in_memory()?;
        connection.busy_timeout(BUSY_TIMEOUT)?;
        configure_resource_limits(&connection)?;
        initialize_schema(&mut connection, Path::new(":memory:"))?;
        configure_in_memory_connection(&connection)?;
        quick_check(&connection)?;
        connection.execute(
            "INSERT INTO metadata(key, value) VALUES (?1, '1')",
            [LEGACY_MIGRATION_KEY],
        )?;
        Ok(Self {
            connection,
            paths,
            persistent_attachments: false,
        })
    }

    pub fn open_default(explicit: Option<PathBuf>) -> Result<Self, StoreError> {
        Self::open(resolve_data_dir(explicit)?)
    }

    pub fn paths(&self) -> &Paths {
        &self.paths
    }

    pub fn data_dir(&self) -> &Path {
        &self.paths.dir
    }

    pub fn images_dir(&self) -> &Path {
        &self.paths.images
    }

    pub fn database_path(&self) -> &Path {
        &self.paths.database
    }

    /// Cheap external-change probe for a long-running TUI.
    pub fn revision(&self) -> Result<u64, StoreError> {
        read_revision(&self.connection)
    }

    pub fn snapshot(&self) -> Result<StoreData, StoreError> {
        let tx = self.connection.unchecked_transaction()?;
        let data = load_snapshot(&tx)?;
        tx.commit()?;
        Ok(data)
    }

    pub fn load_settings(&self) -> Result<Settings, StoreError> {
        Ok(self.snapshot()?.settings)
    }

    pub fn save_settings(&mut self, settings: &Settings) -> Result<(), StoreError> {
        self.update(|data| {
            data.replace_settings(settings.clone())?;
            Ok(())
        })
    }

    /// Run a read-modify-write against a fresh snapshot under
    /// `BEGIN IMMEDIATE`. Every successful call increments `revision` once.
    pub fn update<R>(
        &mut self,
        operation: impl FnOnce(&mut StoreData) -> Result<R, StoreError>,
    ) -> Result<R, StoreError> {
        self.update_with_snapshot(operation)
            .map(|(result, _)| result)
    }

    /// Commit a mutation and return the exact normalized snapshot that was
    /// persisted, including its new revision.
    pub fn update_with_snapshot<R>(
        &mut self,
        operation: impl FnOnce(&mut StoreData) -> Result<R, StoreError>,
    ) -> Result<(R, StoreData), StoreError> {
        self.update_inner(None, operation)
    }

    /// Apply a mutation only if the caller's snapshot is still current.
    pub fn update_if_revision<R>(
        &mut self,
        expected_revision: u64,
        operation: impl FnOnce(&mut StoreData) -> Result<R, StoreError>,
    ) -> Result<R, StoreError> {
        self.update_if_revision_with_snapshot(expected_revision, operation)
            .map(|(result, _)| result)
    }

    pub fn update_if_revision_with_snapshot<R>(
        &mut self,
        expected_revision: u64,
        operation: impl FnOnce(&mut StoreData) -> Result<R, StoreError>,
    ) -> Result<(R, StoreData), StoreError> {
        self.update_inner(Some(expected_revision), operation)
    }

    fn update_inner<R>(
        &mut self,
        expected_revision: Option<u64>,
        operation: impl FnOnce(&mut StoreData) -> Result<R, StoreError>,
    ) -> Result<(R, StoreData), StoreError> {
        let images_root = self
            .persistent_attachments
            .then(|| self.paths.images.clone());
        let tx = self
            .connection
            .transaction_with_behavior(TransactionBehavior::Immediate)?;
        let before = load_snapshot(&tx)?;
        let base_revision = before.revision;
        if let Some(expected) = expected_revision
            && expected != base_revision
        {
            return Err(StoreError::Conflict {
                expected,
                actual: base_revision,
            });
        }
        let mut data = before.clone();
        let result = operation(&mut data)?;
        import_task_attachments(&mut data, images_root.as_deref())?;
        normalize_and_validate(
            &mut data,
            Local::now().naive_local(),
            DueMode::NewWrite,
            AttachmentMode::Persisted,
        )?;
        let next_revision = base_revision
            .checked_add(1)
            .ok_or_else(|| StoreError::Corrupt("revision overflow".into()))?;
        data.revision = next_revision;
        persist_diff(&tx, &before, &data)?;
        tx.commit()?;
        Ok((result, data))
    }

    fn migrate_legacy_json(&mut self) -> Result<(), StoreError> {
        let images_root = self.paths.images.clone();
        let tx = self
            .connection
            .transaction_with_behavior(TransactionBehavior::Immediate)?;
        if migration_complete(&tx)? {
            tx.commit()?;
            return Ok(());
        }

        let existing = load_snapshot(&tx)?;
        if !existing.categories.is_empty()
            || !existing.tasks.is_empty()
            || !existing.attachments.is_empty()
            || existing.revision != 0
        {
            return Err(StoreError::Corrupt(
                "database contains data but has no completed legacy migration marker".into(),
            ));
        }

        let categories_file = read_optional_json::<CategoriesFile>(&self.paths.categories)?;
        let tasks_file = read_optional_json::<TasksFile>(&self.paths.tasks)?;
        let settings = read_optional_json::<Settings>(&self.paths.settings)?;
        validate_legacy_schema(
            &self.paths.categories,
            categories_file.as_ref().map(|f| f.schema),
        )?;
        validate_legacy_schema(&self.paths.tasks, tasks_file.as_ref().map(|f| f.schema))?;

        let has_legacy = categories_file.is_some() || tasks_file.is_some() || settings.is_some();
        if has_legacy {
            let mut data = StoreData {
                revision: 1,
                categories: categories_file
                    .map(|file| {
                        file.categories
                            .into_iter()
                            .filter(|category| !category.is_all())
                            .collect()
                    })
                    .unwrap_or_default(),
                tasks: tasks_file.map(|file| file.tasks).unwrap_or_default(),
                settings: settings.unwrap_or_default().normalized(),
                attachments: Vec::new(),
            };
            import_task_attachments(&mut data, Some(&images_root))?;
            // Legacy relative values used the reader's current date/year. Freeze
            // that interpretation now so it cannot drift after migration.
            normalize_and_validate(
                &mut data,
                Local::now().naive_local(),
                DueMode::LegacyMigration,
                AttachmentMode::Persisted,
            )?;
            persist_diff(&tx, &existing, &data)?;
        }
        tx.execute(
            "INSERT INTO metadata(key, value) VALUES (?1, '1')",
            [LEGACY_MIGRATION_KEY],
        )?;
        tx.commit()?;
        Ok(())
    }
}

pub fn resolve_data_dir(explicit: Option<PathBuf>) -> Result<PathBuf, StoreError> {
    resolve_data_dir_from(
        explicit,
        std::env::var_os("MACH_DIR").map(PathBuf::from),
        dirs::home_dir(),
    )
}

fn resolve_data_dir_from(
    explicit: Option<PathBuf>,
    configured: Option<PathBuf>,
    home: Option<PathBuf>,
) -> Result<PathBuf, StoreError> {
    if let Some(dir) = explicit {
        return expand_user_with_home(dir, home.as_deref());
    }
    if let Some(dir) = configured {
        return expand_user_with_home(dir, home.as_deref());
    }
    home.map(|home| home.join(".mach")).ok_or_else(|| {
        StoreError::validation("could not determine the home directory; use --dir or set MACH_DIR")
    })
}

fn expand_user(path: PathBuf) -> Result<PathBuf, StoreError> {
    let home = dirs::home_dir();
    expand_user_with_home(path, home.as_deref())
}

fn expand_user_with_home(path: PathBuf, home: Option<&Path>) -> Result<PathBuf, StoreError> {
    if path.as_os_str().is_empty() {
        return Err(StoreError::validation("data directory cannot be empty"));
    }
    let text = path.to_string_lossy();
    if text == "~" {
        return home
            .map(Path::to_path_buf)
            .ok_or_else(|| StoreError::validation("cannot expand ~ without a home directory"));
    }
    if let Some(rest) = text.strip_prefix("~/") {
        return home
            .map(|home| home.join(rest))
            .ok_or_else(|| StoreError::validation("cannot expand ~ without a home directory"));
    }
    Ok(path)
}

fn configure_connection(connection: &Connection) -> Result<(), StoreError> {
    connection.pragma_update(None, "foreign_keys", "ON")?;
    connection.pragma_update(None, "synchronous", "FULL")?;
    let mode: String = connection.query_row("PRAGMA journal_mode=WAL", [], |row| row.get(0))?;
    if !mode.eq_ignore_ascii_case("wal") {
        return Err(StoreError::Corrupt(format!(
            "SQLite refused WAL mode (using {mode})"
        )));
    }
    Ok(())
}

fn configure_resource_limits(connection: &Connection) -> Result<(), StoreError> {
    connection.set_limit(Limit::SQLITE_LIMIT_LENGTH, MAX_SQLITE_VALUE_BYTES)?;
    Ok(())
}

fn configure_in_memory_connection(connection: &Connection) -> Result<(), StoreError> {
    connection.pragma_update(None, "foreign_keys", "ON")?;
    connection.pragma_update(None, "journal_mode", "MEMORY")?;
    Ok(())
}

fn initialize_schema(connection: &mut Connection, path: &Path) -> Result<(), StoreError> {
    let version: i64 = connection.query_row("PRAGMA user_version", [], |row| row.get(0))?;
    if version != 0 && version != DATABASE_SCHEMA_VERSION {
        return Err(StoreError::UnsupportedDatabaseSchema {
            path: path.to_path_buf(),
            found: version,
            expected: DATABASE_SCHEMA_VERSION,
        });
    }

    let tx = connection.transaction_with_behavior(TransactionBehavior::Immediate)?;
    tx.execute_batch(
        "
        CREATE TABLE IF NOT EXISTS metadata (
            key TEXT PRIMARY KEY,
            value TEXT NOT NULL
        ) STRICT;
        CREATE TABLE IF NOT EXISTS app_state (
            id INTEGER PRIMARY KEY CHECK (id = 1),
            revision INTEGER NOT NULL CHECK (revision >= 0),
            settings_json TEXT NOT NULL
        ) STRICT;
        CREATE TABLE IF NOT EXISTS categories (
            id TEXT PRIMARY KEY,
            position INTEGER NOT NULL UNIQUE CHECK (position >= 0),
            name TEXT NOT NULL CHECK (length(trim(name)) > 0),
            name_key TEXT NOT NULL UNIQUE CHECK (length(name_key) > 0),
            description TEXT NOT NULL
        ) STRICT;
        CREATE TABLE IF NOT EXISTS tasks (
            id TEXT PRIMARY KEY,
            position INTEGER NOT NULL UNIQUE CHECK (position >= 0),
            title TEXT NOT NULL CHECK (length(trim(title)) > 0),
            body_json TEXT NOT NULL,
            due TEXT NOT NULL,
            created TEXT NOT NULL,
            done INTEGER NOT NULL CHECK (done IN (0, 1)),
            importance INTEGER NOT NULL CHECK (importance BETWEEN 0 AND 3),
            category_id TEXT REFERENCES categories(id) ON DELETE SET NULL
        ) STRICT;
        CREATE TABLE IF NOT EXISTS attachments (
            id TEXT PRIMARY KEY,
            sha256 TEXT NOT NULL UNIQUE CHECK (sha256 = id),
            media_type TEXT NOT NULL,
            byte_len INTEGER NOT NULL CHECK (byte_len > 0),
            storage_name TEXT NOT NULL UNIQUE
        ) STRICT;
        CREATE TABLE IF NOT EXISTS task_attachments (
            task_id TEXT NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
            block_index INTEGER NOT NULL CHECK (block_index >= 0),
            attachment_id TEXT NOT NULL REFERENCES attachments(id) ON DELETE RESTRICT,
            PRIMARY KEY (task_id, block_index)
        ) STRICT;
        CREATE INDEX IF NOT EXISTS task_attachments_by_attachment
            ON task_attachments(attachment_id);
        ",
    )?;
    let settings = serde_json::to_string(&Settings::default()).map_err(|source| {
        StoreError::Corrupt(format!("could not encode default settings: {source}"))
    })?;
    tx.execute(
        "INSERT OR IGNORE INTO app_state(id, revision, settings_json) VALUES (1, 0, ?1)",
        [settings],
    )?;
    if version == 0 {
        tx.pragma_update(None, "user_version", DATABASE_SCHEMA_VERSION)?;
    }
    tx.commit()?;
    Ok(())
}

fn quick_check(connection: &Connection) -> Result<(), StoreError> {
    let result: String = connection.query_row("PRAGMA quick_check(1)", [], |row| row.get(0))?;
    if result != "ok" {
        return Err(StoreError::Corrupt(format!(
            "SQLite quick check failed: {result}"
        )));
    }
    Ok(())
}

fn migration_complete(connection: &Connection) -> Result<bool, StoreError> {
    let value: Option<String> = connection
        .query_row(
            "SELECT value FROM metadata WHERE key = ?1",
            [LEGACY_MIGRATION_KEY],
            |row| row.get(0),
        )
        .optional()?;
    Ok(value.as_deref() == Some("1"))
}

fn read_revision(connection: &Connection) -> Result<u64, StoreError> {
    let value: i64 =
        connection.query_row("SELECT revision FROM app_state WHERE id = 1", [], |row| {
            row.get(0)
        })?;
    u64::try_from(value).map_err(|_| StoreError::Corrupt(format!("negative revision {value}")))
}

fn load_snapshot(connection: &Connection) -> Result<StoreData, StoreError> {
    let (revision, settings_json): (i64, String) = connection.query_row(
        "SELECT revision, settings_json FROM app_state WHERE id = 1",
        [],
        |row| Ok((row.get(0)?, row.get(1)?)),
    )?;
    let revision = u64::try_from(revision)
        .map_err(|_| StoreError::Corrupt(format!("negative revision {revision}")))?;
    let settings: Settings = serde_json::from_str(&settings_json)
        .map_err(|error| StoreError::Corrupt(format!("invalid settings JSON: {error}")))?;

    let mut attachment_statement = connection.prepare(
        "SELECT id, sha256, media_type, byte_len, storage_name FROM attachments ORDER BY id",
    )?;
    let attachment_rows = attachment_statement.query_map([], |row| {
        Ok((
            row.get::<_, String>(0)?,
            row.get::<_, String>(1)?,
            row.get::<_, String>(2)?,
            row.get::<_, i64>(3)?,
            row.get::<_, String>(4)?,
        ))
    })?;
    let mut attachments = Vec::new();
    for row in attachment_rows {
        let (id, sha256, media_type, byte_len, storage_name) = row?;
        attachments.push(Attachment {
            id,
            sha256,
            media_type,
            byte_len: u64::try_from(byte_len).map_err(|_| {
                StoreError::Corrupt(format!("attachment has invalid byte length {byte_len}"))
            })?,
            storage_name,
        });
    }

    let mut categories_statement = connection.prepare(
        "SELECT position, id, name, name_key, description FROM categories ORDER BY position",
    )?;
    let category_rows = categories_statement.query_map([], |row| {
        Ok((
            row.get::<_, i64>(0)?,
            Category {
                id: row.get(1)?,
                name: row.get(2)?,
                description: row.get(4)?,
            },
            row.get::<_, String>(3)?,
        ))
    })?;
    let mut categories = Vec::new();
    for (expected_position, row) in category_rows.enumerate() {
        if expected_position >= MAX_CATEGORY_COUNT {
            return Err(StoreError::Corrupt(format!(
                "category count exceeds {MAX_CATEGORY_COUNT}"
            )));
        }
        let (stored_position, category, stored_name_key) = row?;
        validate_stored_position(stored_position, expected_position, "category")?;
        let expected_name_key = category_name_key(&category.name);
        if stored_name_key != expected_name_key {
            return Err(StoreError::Corrupt(format!(
                "category {:?} has identity key {stored_name_key:?}, expected {expected_name_key:?}",
                category.id
            )));
        }
        categories.push(category);
    }

    let mut tasks_statement = connection.prepare(
        "SELECT position, id, title, body_json, due, created, done, importance, category_id
         FROM tasks ORDER BY position",
    )?;
    let rows = tasks_statement.query_map([], |row| {
        Ok((
            row.get::<_, i64>(0)?,
            row.get::<_, String>(1)?,
            row.get::<_, String>(2)?,
            row.get::<_, String>(3)?,
            row.get::<_, String>(4)?,
            row.get::<_, String>(5)?,
            row.get::<_, i64>(6)?,
            row.get::<_, i64>(7)?,
            row.get::<_, Option<String>>(8)?,
        ))
    })?;
    let mut tasks = Vec::new();
    for (expected_position, row) in rows.enumerate() {
        if expected_position >= MAX_TASK_COUNT {
            return Err(StoreError::Corrupt(format!(
                "task count exceeds {MAX_TASK_COUNT}"
            )));
        }
        let (stored_position, id, title, body_json, due, created, done, importance, category_id) =
            row?;
        validate_stored_position(stored_position, expected_position, "task")?;
        let body = serde_json::from_str::<Vec<Block>>(&body_json).map_err(|error| {
            StoreError::Corrupt(format!("task {id:?} has invalid body JSON: {error}"))
        })?;
        let importance = u8::try_from(importance).map_err(|_| {
            StoreError::Corrupt(format!("task {id:?} has invalid importance {importance}"))
        })?;
        tasks.push(Task {
            id,
            title,
            body,
            due,
            created,
            done: done != 0,
            importance,
            category_id,
        });
    }
    validate_task_attachment_rows(connection, &tasks)?;
    let mut data = StoreData {
        revision,
        categories,
        tasks,
        settings,
        attachments,
    };
    normalize_and_validate(
        &mut data,
        Local::now().naive_local(),
        DueMode::Stored,
        AttachmentMode::Persisted,
    )
    .map_err(|error| match error {
        StoreError::Validation(message) => StoreError::Corrupt(message),
        other => other,
    })?;
    Ok(data)
}

fn validate_task_attachment_rows(
    connection: &Connection,
    tasks: &[Task],
) -> Result<(), StoreError> {
    let expected: HashSet<(String, usize, String)> = tasks
        .iter()
        .flat_map(|task| {
            task.body
                .iter()
                .enumerate()
                .filter_map(|(block_index, block)| match block {
                    Block::Image { attachment_id } => {
                        Some((task.id.clone(), block_index, attachment_id.clone()))
                    }
                    _ => None,
                })
        })
        .collect();
    let mut statement = connection.prepare(
        "SELECT task_id, block_index, attachment_id
         FROM task_attachments ORDER BY task_id, block_index",
    )?;
    let rows = statement.query_map([], |row| {
        Ok((
            row.get::<_, String>(0)?,
            row.get::<_, i64>(1)?,
            row.get::<_, String>(2)?,
        ))
    })?;
    let mut stored = HashSet::new();
    for row in rows {
        let (task_id, block_index, attachment_id) = row?;
        let block_index = usize::try_from(block_index).map_err(|_| {
            StoreError::Corrupt(format!(
                "task {task_id:?} has invalid attachment reference index {block_index}"
            ))
        })?;
        stored.insert((task_id, block_index, attachment_id));
    }
    if stored != expected {
        return Err(StoreError::Corrupt(
            "task attachment reference rows do not match task body JSON".into(),
        ));
    }
    Ok(())
}

fn validate_stored_position(stored: i64, expected: usize, entity: &str) -> Result<(), StoreError> {
    let expected = i64::try_from(expected)
        .map_err(|_| StoreError::Corrupt(format!("{entity} position exceeds integer range")))?;
    if stored != expected {
        return Err(StoreError::Corrupt(format!(
            "{entity} position {stored} is not contiguous (expected {expected})"
        )));
    }
    Ok(())
}

/// Persist only rows whose identity, content, or position changed.
///
/// Positions and category names have UNIQUE constraints. Rows that move or
/// change names are first assigned transaction-private values outside the
/// validated application domain, which makes swaps and insertions safe without
/// deleting and recreating unrelated rows.
fn persist_diff(
    tx: &Transaction<'_>,
    before: &StoreData,
    after: &StoreData,
) -> Result<(), StoreError> {
    let before_categories: HashMap<&str, (usize, &Category)> = before
        .categories
        .iter()
        .enumerate()
        .map(|(position, category)| (category.id.as_str(), (position, category)))
        .collect();
    let after_categories: HashMap<&str, (usize, &Category)> = after
        .categories
        .iter()
        .enumerate()
        .map(|(position, category)| (category.id.as_str(), (position, category)))
        .collect();
    let before_tasks: HashMap<&str, (usize, &Task)> = before
        .tasks
        .iter()
        .enumerate()
        .map(|(position, task)| (task.id.as_str(), (position, task)))
        .collect();
    let after_tasks: HashMap<&str, (usize, &Task)> = after
        .tasks
        .iter()
        .enumerate()
        .map(|(position, task)| (task.id.as_str(), (position, task)))
        .collect();
    let before_attachments: HashMap<&str, &Attachment> = before
        .attachments
        .iter()
        .map(|attachment| (attachment.id.as_str(), attachment))
        .collect();
    let after_attachments: HashMap<&str, &Attachment> = after
        .attachments
        .iter()
        .map(|attachment| (attachment.id.as_str(), attachment))
        .collect();

    for attachment in &before.attachments {
        if after_attachments.get(attachment.id.as_str()).copied() != Some(attachment) {
            return Err(StoreError::Validation(format!(
                "attachment {:?} metadata is immutable",
                attachment.id
            )));
        }
    }
    for attachment in &after.attachments {
        if !before_attachments.contains_key(attachment.id.as_str()) {
            tx.execute(
                "INSERT INTO attachments(id, sha256, media_type, byte_len, storage_name)
                 VALUES (?1, ?2, ?3, ?4, ?5)",
                params![
                    attachment.id,
                    attachment.sha256,
                    attachment.media_type,
                    sqlite_attachment_size(attachment.byte_len)?,
                    attachment.storage_name,
                ],
            )?;
        }
    }

    // Remove tasks first so deleting a task and its category does not produce
    // an unnecessary ON DELETE SET NULL update.
    for task in &before.tasks {
        if !after_tasks.contains_key(task.id.as_str()) {
            execute_one(
                tx,
                "DELETE FROM tasks WHERE id = ?1",
                [task.id.as_str()],
                "task",
                &task.id,
            )?;
        }
    }

    // Free every old identity key that may be replaced. Control characters are
    // rejected by validation, so these temporary values cannot collide with
    // application data and are never visible outside this transaction.
    let mut temporary_name_index = 0usize;
    for category in &before.categories {
        let name_changed_or_removed = after_categories
            .get(category.id.as_str())
            .is_none_or(|(_, current)| current.name != category.name);
        if name_changed_or_removed {
            let temporary_name = format!("\u{1f}mach-category-{temporary_name_index}");
            temporary_name_index += 1;
            execute_one(
                tx,
                "UPDATE categories SET name = ?1, name_key = ?1 WHERE id = ?2",
                params![temporary_name, category.id],
                "category",
                &category.id,
            )?;
        }
    }

    let category_position_base = before.categories.len().max(after.categories.len());
    let mut category_position_offset = 0usize;
    for (old_position, category) in before.categories.iter().enumerate() {
        if let Some((new_position, _)) = after_categories.get(category.id.as_str()).copied()
            && new_position != old_position
        {
            let temporary = temporary_position(
                category_position_base,
                category_position_offset,
                "categories",
            )?;
            category_position_offset += 1;
            execute_one(
                tx,
                "UPDATE categories SET position = ?1 WHERE id = ?2",
                params![temporary, category.id],
                "category",
                &category.id,
            )?;
        }
    }
    for category in &after.categories {
        if !before_categories.contains_key(category.id.as_str()) {
            let temporary = temporary_position(
                category_position_base,
                category_position_offset,
                "categories",
            )?;
            category_position_offset += 1;
            tx.execute(
                "INSERT INTO categories(id, position, name, name_key, description)
                 VALUES (?1, ?2, ?3, ?4, ?5)",
                params![
                    category.id,
                    temporary,
                    category.name,
                    category_name_key(&category.name),
                    category.description
                ],
            )?;
        }
    }

    let task_position_base = before.tasks.len().max(after.tasks.len());
    let mut task_position_offset = 0usize;
    for (old_position, task) in before.tasks.iter().enumerate() {
        if let Some((new_position, _)) = after_tasks.get(task.id.as_str()).copied()
            && new_position != old_position
        {
            let temporary = temporary_position(task_position_base, task_position_offset, "tasks")?;
            task_position_offset += 1;
            execute_one(
                tx,
                "UPDATE tasks SET position = ?1 WHERE id = ?2",
                params![temporary, task.id],
                "task",
                &task.id,
            )?;
        }
    }

    // New categories now exist, so task foreign keys can safely move to them
    // before obsolete categories are deleted.
    for task in &after.tasks {
        if let Some((_, previous)) = before_tasks.get(task.id.as_str()).copied()
            && previous != task
        {
            let body_json = encode_task_body(task)?;
            execute_one(
                tx,
                "UPDATE tasks SET
                    title = ?1, body_json = ?2, due = ?3, created = ?4,
                    done = ?5, importance = ?6, category_id = ?7
                 WHERE id = ?8",
                params![
                    task.title,
                    body_json,
                    task.due,
                    task.created,
                    i64::from(task.done),
                    i64::from(task.importance),
                    task.category_id,
                    task.id,
                ],
                "task",
                &task.id,
            )?;
        }
    }
    for task in &after.tasks {
        if !before_tasks.contains_key(task.id.as_str()) {
            let temporary = temporary_position(task_position_base, task_position_offset, "tasks")?;
            task_position_offset += 1;
            let body_json = encode_task_body(task)?;
            tx.execute(
                "INSERT INTO tasks(
                    id, position, title, body_json, due, created, done, importance, category_id
                 ) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9)",
                params![
                    task.id,
                    temporary,
                    task.title,
                    body_json,
                    task.due,
                    task.created,
                    i64::from(task.done),
                    i64::from(task.importance),
                    task.category_id,
                ],
            )?;
        }
    }

    for task in &after.tasks {
        let body_changed_or_new = before_tasks
            .get(task.id.as_str())
            .is_none_or(|(_, previous)| previous.body != task.body);
        if body_changed_or_new {
            tx.execute(
                "DELETE FROM task_attachments WHERE task_id = ?1",
                [&task.id],
            )?;
            insert_task_attachment_rows(tx, task)?;
        }
    }

    for category in &before.categories {
        if !after_categories.contains_key(category.id.as_str()) {
            execute_one(
                tx,
                "DELETE FROM categories WHERE id = ?1",
                [category.id.as_str()],
                "category",
                &category.id,
            )?;
        }
    }

    for category in &after.categories {
        if let Some((_, previous)) = before_categories.get(category.id.as_str()).copied()
            && previous != category
        {
            execute_one(
                tx,
                "UPDATE categories
                 SET name = ?1, name_key = ?2, description = ?3
                 WHERE id = ?4",
                params![
                    category.name,
                    category_name_key(&category.name),
                    category.description,
                    category.id
                ],
                "category",
                &category.id,
            )?;
        }
    }

    // All rows whose final slots changed are currently at unique temporary
    // positions. Rows omitted here kept the same slot, so final assignment
    // cannot collide with them.
    for (position, category) in after.categories.iter().enumerate() {
        let moved_or_new = before_categories
            .get(category.id.as_str())
            .is_none_or(|(old_position, _)| *old_position != position);
        if moved_or_new {
            let position = sqlite_position(position, "categories")?;
            execute_one(
                tx,
                "UPDATE categories SET position = ?1 WHERE id = ?2",
                params![position, category.id],
                "category",
                &category.id,
            )?;
        }
    }
    for (position, task) in after.tasks.iter().enumerate() {
        let moved_or_new = before_tasks
            .get(task.id.as_str())
            .is_none_or(|(old_position, _)| *old_position != position);
        if moved_or_new {
            let position = sqlite_position(position, "tasks")?;
            execute_one(
                tx,
                "UPDATE tasks SET position = ?1 WHERE id = ?2",
                params![position, task.id],
                "task",
                &task.id,
            )?;
        }
    }

    let settings = (before.settings != after.settings).then_some(&after.settings);
    persist_app_state(tx, after.revision, settings)?;
    Ok(())
}

fn execute_one<P: rusqlite::Params>(
    tx: &Transaction<'_>,
    sql: &str,
    params: P,
    entity: &str,
    id: &str,
) -> Result<(), StoreError> {
    let changed = tx.execute(sql, params)?;
    if changed != 1 {
        return Err(StoreError::Corrupt(format!(
            "expected to change one {entity} {id:?}, changed {changed}"
        )));
    }
    Ok(())
}

fn temporary_position(base: usize, offset: usize, entity: &str) -> Result<i64, StoreError> {
    let position = base
        .checked_add(offset)
        .ok_or_else(|| StoreError::Validation(format!("too many {entity}")))?;
    sqlite_position(position, entity)
}

fn sqlite_position(position: usize, entity: &str) -> Result<i64, StoreError> {
    i64::try_from(position).map_err(|_| StoreError::Validation(format!("too many {entity}")))
}

fn sqlite_attachment_size(byte_len: u64) -> Result<i64, StoreError> {
    i64::try_from(byte_len)
        .map_err(|_| StoreError::Validation("attachment byte length exceeds integer range".into()))
}

fn insert_task_attachment_rows(tx: &Transaction<'_>, task: &Task) -> Result<(), StoreError> {
    let mut statement = tx.prepare(
        "INSERT INTO task_attachments(task_id, block_index, attachment_id)
         VALUES (?1, ?2, ?3)",
    )?;
    for (block_index, block) in task.body.iter().enumerate() {
        let Block::Image { attachment_id } = block else {
            continue;
        };
        statement.execute(params![
            task.id,
            sqlite_position(block_index, "task attachment blocks")?,
            attachment_id,
        ])?;
    }
    Ok(())
}

fn encode_task_body(task: &Task) -> Result<String, StoreError> {
    serde_json::to_string(&task.body).map_err(|error| {
        StoreError::Corrupt(format!("could not encode task {:?}: {error}", task.id))
    })
}

fn persist_app_state(
    tx: &Transaction<'_>,
    revision: u64,
    settings: Option<&Settings>,
) -> Result<(), StoreError> {
    let revision = i64::try_from(revision)
        .map_err(|_| StoreError::Corrupt("revision exceeds SQLite integer range".into()))?;
    let changed = if let Some(settings) = settings {
        let settings_json = serde_json::to_string(settings)
            .map_err(|error| StoreError::Corrupt(format!("could not encode settings: {error}")))?;
        tx.execute(
            "UPDATE app_state SET revision = ?1, settings_json = ?2 WHERE id = 1",
            params![revision, settings_json],
        )?
    } else {
        tx.execute(
            "UPDATE app_state SET revision = ?1 WHERE id = 1",
            [revision],
        )?
    };
    if changed != 1 {
        return Err(StoreError::Corrupt(format!(
            "expected to update app state, changed {changed} rows"
        )));
    }
    Ok(())
}

fn import_task_attachments(
    data: &mut StoreData,
    images_root: Option<&Path>,
) -> Result<(), StoreError> {
    let mut known: HashMap<String, Attachment> = data
        .attachments
        .iter()
        .cloned()
        .map(|attachment| (attachment.id.clone(), attachment))
        .collect();

    for task in &mut data.tasks {
        for block in &mut task.body {
            let Block::Image { attachment_id } = block else {
                continue;
            };
            if known.contains_key(attachment_id) {
                continue;
            }
            if is_attachment_id(attachment_id) {
                return Err(StoreError::Validation(format!(
                    "task {:?} refers to unknown attachment {attachment_id:?}",
                    task.id
                )));
            }
            let Some(images_root) = images_root else {
                return Err(StoreError::Validation(
                    "image attachments require a persistent store".into(),
                ));
            };
            let imported = import_attachment(attachment_id, images_root)?;
            if let Some(existing) = known.get(&imported.id) {
                if existing != &imported {
                    return Err(StoreError::Corrupt(format!(
                        "attachment {:?} metadata does not match imported content",
                        imported.id
                    )));
                }
            } else {
                known.insert(imported.id.clone(), imported.clone());
                data.attachments.push(imported.clone());
            }
            *attachment_id = imported.id;
        }
    }
    data.attachments
        .sort_by(|left, right| left.id.cmp(&right.id));
    Ok(())
}

fn import_attachment(reference: &str, images_root: &Path) -> Result<Attachment, StoreError> {
    let source_path = crate::image::expand_in(reference, images_root);
    let mut source = fs::File::open(&source_path)
        .map_err(|error| StoreError::io("open image attachment", &source_path, error))?;
    let metadata = source
        .metadata()
        .map_err(|error| StoreError::io("inspect image attachment", &source_path, error))?;
    if !metadata.is_file() {
        return Err(StoreError::Validation(format!(
            "image attachment {} is not a regular file",
            source_path.display()
        )));
    }

    ensure_private_directory(images_root)?;
    let temp_path = images_root.join(format!(".mach-attachment-{}.tmp", uuid::Uuid::new_v4()));
    let mut temp = open_private_attachment_temp(&temp_path)?;
    let result = (|| {
        let mut hasher = Sha256::new();
        let mut byte_len = 0_u64;
        let mut prefix = [0_u8; 32];
        let mut prefix_len = 0usize;
        let mut buffer = [0_u8; 64 * 1024];
        loop {
            let read = source
                .read(&mut buffer)
                .map_err(|error| StoreError::io("read image attachment", &source_path, error))?;
            if read == 0 {
                break;
            }
            byte_len = byte_len
                .checked_add(read as u64)
                .ok_or_else(|| StoreError::Validation("image attachment is too large".into()))?;
            if byte_len > MAX_ATTACHMENT_BYTES {
                return Err(StoreError::Validation(format!(
                    "image attachment {} exceeds the {} MiB safety limit",
                    source_path.display(),
                    MAX_ATTACHMENT_BYTES / 1024 / 1024
                )));
            }
            if prefix_len < prefix.len() {
                let copy = (prefix.len() - prefix_len).min(read);
                prefix[prefix_len..prefix_len + copy].copy_from_slice(&buffer[..copy]);
                prefix_len += copy;
            }
            hasher.update(&buffer[..read]);
            temp.write_all(&buffer[..read]).map_err(|error| {
                StoreError::io("write managed image attachment", &temp_path, error)
            })?;
        }
        if byte_len == 0 {
            return Err(StoreError::Validation(format!(
                "image attachment {} is empty",
                source_path.display()
            )));
        }
        let format = image::guess_format(&prefix[..prefix_len]).map_err(|_| {
            StoreError::Validation(format!(
                "image attachment {} is not a supported PNG, JPEG, GIF, or WebP image",
                source_path.display()
            ))
        })?;
        let (extension, media_type) = attachment_format(format).ok_or_else(|| {
            StoreError::Validation(format!(
                "image attachment {} is not a supported PNG, JPEG, GIF, or WebP image",
                source_path.display()
            ))
        })?;
        temp.sync_all()
            .map_err(|error| StoreError::io("sync managed image attachment", &temp_path, error))?;
        drop(temp);
        crate::image::load_dynamic(&temp_path).map_err(StoreError::Validation)?;

        let id = format!("{:x}", hasher.finalize());
        let storage_name = format!("{id}.{extension}");
        let destination = images_root.join(&storage_name);
        if destination.exists() {
            let (stored_hash, stored_len) = hash_attachment_file(&destination)?;
            if stored_hash != id || stored_len != byte_len {
                return Err(StoreError::Corrupt(format!(
                    "managed attachment {} does not match its content address",
                    destination.display()
                )));
            }
            fs::remove_file(&temp_path).map_err(|error| {
                StoreError::io("remove duplicate image attachment", &temp_path, error)
            })?;
        } else {
            fs::rename(&temp_path, &destination).map_err(|error| {
                StoreError::io("install managed image attachment", &destination, error)
            })?;
            set_private_file(&destination)?;
            fs::File::open(images_root)
                .and_then(|directory| directory.sync_all())
                .map_err(|error| StoreError::io("sync image directory", images_root, error))?;
        }
        Ok(Attachment {
            id: id.clone(),
            sha256: id,
            media_type: media_type.into(),
            byte_len,
            storage_name,
        })
    })();
    if result.is_err() {
        let _ = fs::remove_file(&temp_path);
    }
    result
}

fn open_private_attachment_temp(path: &Path) -> Result<fs::File, StoreError> {
    let mut options = fs::OpenOptions::new();
    options.write(true).create_new(true);
    #[cfg(unix)]
    {
        use std::os::unix::fs::OpenOptionsExt;
        options.mode(0o600);
    }
    options
        .open(path)
        .map_err(|error| StoreError::io("create managed image attachment", path, error))
}

fn hash_attachment_file(path: &Path) -> Result<(String, u64), StoreError> {
    let mut file = fs::File::open(path)
        .map_err(|error| StoreError::io("open managed image attachment", path, error))?;
    let mut hasher = Sha256::new();
    let mut byte_len = 0_u64;
    let mut buffer = [0_u8; 64 * 1024];
    loop {
        let read = file
            .read(&mut buffer)
            .map_err(|error| StoreError::io("read managed image attachment", path, error))?;
        if read == 0 {
            break;
        }
        byte_len = byte_len
            .checked_add(read as u64)
            .ok_or_else(|| StoreError::Corrupt("managed attachment is too large".into()))?;
        if byte_len > MAX_ATTACHMENT_BYTES {
            return Err(StoreError::Corrupt(format!(
                "managed attachment {} exceeds the safety limit",
                path.display()
            )));
        }
        hasher.update(&buffer[..read]);
    }
    Ok((format!("{:x}", hasher.finalize()), byte_len))
}

fn attachment_format(format: ImageFormat) -> Option<(&'static str, &'static str)> {
    match format {
        ImageFormat::Png => Some(("png", "image/png")),
        ImageFormat::Jpeg => Some(("jpg", "image/jpeg")),
        ImageFormat::Gif => Some(("gif", "image/gif")),
        ImageFormat::WebP => Some(("webp", "image/webp")),
        _ => None,
    }
}

fn is_attachment_id(value: &str) -> bool {
    value.len() == ATTACHMENT_ID_LEN
        && value
            .bytes()
            .all(|byte| byte.is_ascii_digit() || (b'a'..=b'f').contains(&byte))
}

#[derive(Clone, Copy)]
enum DueMode {
    NewWrite,
    LegacyMigration,
    Stored,
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum AttachmentMode {
    Draft,
    Persisted,
}

fn normalize_and_validate(
    data: &mut StoreData,
    now: NaiveDateTime,
    due_mode: DueMode,
    attachment_mode: AttachmentMode,
) -> Result<(), StoreError> {
    if data.categories.len() > MAX_CATEGORY_COUNT {
        return Err(StoreError::Validation(format!(
            "category limit is {MAX_CATEGORY_COUNT}"
        )));
    }
    if data.tasks.len() > MAX_TASK_COUNT {
        return Err(StoreError::Validation(format!(
            "task limit is {MAX_TASK_COUNT}"
        )));
    }

    let attachment_ids = validate_attachments(&data.attachments)?;

    let mut category_ids = HashSet::new();
    let mut category_names = HashSet::new();
    for category in &data.categories {
        validate_single_line(&category.id, "category id")?;
        validate_byte_limit(&category.id, ID_MAX_BYTES, "category id")?;
        if category.is_all() {
            return Err(StoreError::Validation(
                "real category id cannot be empty".into(),
            ));
        }
        if !category_ids.insert(category.id.as_str()) {
            return Err(StoreError::Validation(format!(
                "category id {:?} must be unique",
                category.id
            )));
        }
        validate_single_line(&category.name, "category name")?;
        validate_byte_limit(
            &category.name,
            text_byte_limit(MAX_CATEGORY_NAME_LEN),
            "category name",
        )?;
        let name = category.name.trim();
        if name.is_empty() {
            return Err(StoreError::Validation(
                "category name cannot be empty".into(),
            ));
        }
        if name.graphemes(true).count() > MAX_CATEGORY_NAME_LEN {
            return Err(StoreError::Validation(format!(
                "category name {:?} exceeds {MAX_CATEGORY_NAME_LEN} characters",
                category.name
            )));
        }
        if !category_names.insert(category_name_key(name)) {
            return Err(StoreError::Validation(format!(
                "category names must be unique (duplicate {:?})",
                category.name
            )));
        }
        validate_multiline(
            &category.description,
            MAX_CATEGORY_DESC_LINES,
            MAX_CATEGORY_DESC_LINE_LEN,
            "category description",
        )?;
    }

    let mut task_ids = HashSet::new();
    for task in &mut data.tasks {
        validate_single_line(&task.id, "task id")?;
        validate_byte_limit(&task.id, ID_MAX_BYTES, "task id")?;
        if task.id.is_empty() || !task_ids.insert(task.id.as_str()) {
            return Err(StoreError::Validation(format!(
                "task id {:?} must be nonempty and unique",
                task.id
            )));
        }
        validate_single_line(&task.title, "task title")?;
        validate_byte_limit(&task.title, text_byte_limit(MAX_TITLE_LEN), "task title")?;
        if task.title.trim().is_empty() {
            return Err(StoreError::Validation(format!(
                "task {:?} title cannot be empty",
                task.id
            )));
        }
        if task.title.graphemes(true).count() > MAX_TITLE_LEN {
            return Err(StoreError::Validation(format!(
                "task {:?} title exceeds {MAX_TITLE_LEN} characters",
                task.id
            )));
        }
        if task.importance > MAX_IMPORTANCE {
            return Err(StoreError::Validation(format!(
                "task {:?} importance must be 0-{MAX_IMPORTANCE}",
                task.id
            )));
        }
        if task.body.len() > MAX_BODY_LINES {
            return Err(StoreError::Validation(format!(
                "task {:?} body exceeds {MAX_BODY_LINES} blocks",
                task.id
            )));
        }
        for block in &task.body {
            validate_block(block, &task.id)?;
            if let Block::Image { attachment_id } = block {
                let known = attachment_ids.contains(attachment_id.as_str());
                if attachment_mode == AttachmentMode::Persisted && !known {
                    return Err(StoreError::Validation(format!(
                        "task {:?} refers to unknown attachment {attachment_id:?}",
                        task.id
                    )));
                }
                if attachment_mode == AttachmentMode::Draft
                    && is_attachment_id(attachment_id)
                    && !known
                {
                    return Err(StoreError::Validation(format!(
                        "task {:?} refers to unknown attachment {attachment_id:?}",
                        task.id
                    )));
                }
            }
        }
        if let Some(category_id) = task.category_id.as_deref() {
            validate_single_line(category_id, "task category id")?;
            validate_byte_limit(category_id, ID_MAX_BYTES, "task category id")?;
            if !category_ids.contains(category_id) {
                return Err(StoreError::Validation(format!(
                    "task {:?} refers to unknown category {category_id:?}",
                    task.id
                )));
            }
        }
        validate_single_line(&task.due, "task due")?;
        validate_byte_limit(&task.due, DUE_MAX_BYTES, "task due")?;
        let normalized_due = match due_mode {
            DueMode::NewWrite => due::normalize_for_write_at(&task.due, now),
            DueMode::LegacyMigration => due::normalize_legacy_at(&task.due, now),
            DueMode::Stored => due::normalize_for_write_at(&task.due, now),
        }
        .map_err(|error| StoreError::Validation(format!("task {:?} has {error}", task.id)))?;
        if matches!(due_mode, DueMode::Stored) && normalized_due != task.due {
            return Err(StoreError::Validation(format!(
                "task {:?} has noncanonical due value {:?}",
                task.id, task.due
            )));
        }
        task.due = normalized_due;
        validate_single_line(&task.created, "task creation timestamp")?;
        validate_byte_limit(&task.created, CREATED_MAX_BYTES, "task creation timestamp")?;
        NaiveDateTime::parse_from_str(&task.created, "%Y-%m-%d %H:%M:%S").map_err(|_| {
            StoreError::Validation(format!(
                "task {:?} has invalid creation timestamp {:?}",
                task.id, task.created
            ))
        })?;
    }
    validate_settings(&data.settings)
}

fn validate_block(block: &Block, task_id: &str) -> Result<(), StoreError> {
    let (kind, value) = match block {
        Block::Text { text } => ("text", text),
        Block::Todo { text, .. } => ("subtask", text),
        Block::Bullet { text } => ("bullet", text),
        Block::Number { text } => ("number", text),
        Block::Link { url } => ("link", url),
        Block::Image { attachment_id } => ("image attachment", attachment_id),
    };
    validate_single_line(value, kind)?;
    validate_byte_limit(value, text_byte_limit(MAX_NOTES_LINE_LEN), kind)?;
    if value.graphemes(true).count() > MAX_NOTES_LINE_LEN {
        return Err(StoreError::Validation(format!(
            "task {task_id:?} {kind} exceeds {MAX_NOTES_LINE_LEN} characters"
        )));
    }
    Ok(())
}

fn validate_attachments(attachments: &[Attachment]) -> Result<HashSet<&str>, StoreError> {
    let mut ids = HashSet::new();
    let mut storage_names = HashSet::new();
    for attachment in attachments {
        if !is_attachment_id(&attachment.id) || attachment.sha256 != attachment.id {
            return Err(StoreError::Validation(format!(
                "attachment {:?} has an invalid content address",
                attachment.id
            )));
        }
        if !ids.insert(attachment.id.as_str()) {
            return Err(StoreError::Validation(format!(
                "attachment id {:?} must be unique",
                attachment.id
            )));
        }
        if attachment.byte_len == 0 || attachment.byte_len > MAX_ATTACHMENT_BYTES {
            return Err(StoreError::Validation(format!(
                "attachment {:?} has invalid byte length {}",
                attachment.id, attachment.byte_len
            )));
        }
        let extension = match attachment.media_type.as_str() {
            "image/png" => "png",
            "image/jpeg" => "jpg",
            "image/gif" => "gif",
            "image/webp" => "webp",
            other => {
                return Err(StoreError::Validation(format!(
                    "attachment {:?} has unsupported media type {other:?}",
                    attachment.id
                )));
            }
        };
        let expected_storage_name = format!("{}.{}", attachment.id, extension);
        if attachment.storage_name != expected_storage_name {
            return Err(StoreError::Validation(format!(
                "attachment {:?} has invalid storage name {:?}",
                attachment.id, attachment.storage_name
            )));
        }
        if !storage_names.insert(attachment.storage_name.as_str()) {
            return Err(StoreError::Validation(format!(
                "attachment storage name {:?} must be unique",
                attachment.storage_name
            )));
        }
    }
    Ok(ids)
}

fn validate_multiline(
    value: &str,
    max_lines: usize,
    max_line_len: usize,
    label: &str,
) -> Result<(), StoreError> {
    let max_line_bytes = text_byte_limit(max_line_len);
    let max_total_bytes = max_lines.saturating_mul(max_line_bytes.saturating_add(1));
    validate_byte_limit(value, max_total_bytes, label)?;
    if value
        .chars()
        .any(|character| character.is_control() && character != '\n')
    {
        return Err(StoreError::Validation(format!(
            "{label} contains a control character"
        )));
    }
    for (index, line) in value.split('\n').enumerate() {
        if index >= max_lines {
            return Err(StoreError::Validation(format!(
                "{label} exceeds {max_lines} lines"
            )));
        }
        if line.len() > max_line_bytes {
            return Err(StoreError::Validation(format!(
                "{label} line exceeds {max_line_bytes} bytes"
            )));
        }
        if line.graphemes(true).count() > max_line_len {
            return Err(StoreError::Validation(format!(
                "{label} line exceeds {max_line_len} characters"
            )));
        }
    }
    Ok(())
}

fn validate_single_line(value: &str, label: &str) -> Result<(), StoreError> {
    if value.chars().any(char::is_control) {
        return Err(StoreError::Validation(format!(
            "{label} contains a control character"
        )));
    }
    Ok(())
}

fn validate_byte_limit(value: &str, max_bytes: usize, label: &str) -> Result<(), StoreError> {
    if value.len() > max_bytes {
        return Err(StoreError::Validation(format!(
            "{label} exceeds {max_bytes} bytes"
        )));
    }
    Ok(())
}

/// Unicode compatibility normalization followed by full default case folding
/// defines category identity. A final normalization makes the key stable when
/// folding introduces decomposed characters.
fn category_name_key(value: &str) -> String {
    caseless_key(value.trim())
}

fn category_name_has_prefix(name: &str, folded_query: &str) -> bool {
    let normalized: String = name.trim().nfkc().collect();
    normalized
        .char_indices()
        .skip(1)
        .map(|(index, _)| index)
        .chain(std::iter::once(normalized.len()))
        .any(|end| category_name_key(&normalized[..end]) == folded_query)
}

fn validate_settings(settings: &Settings) -> Result<(), StoreError> {
    validate_single_line(&settings.date_format, "date format")?;
    validate_byte_limit(
        &settings.date_format,
        SETTINGS_VALUE_MAX_BYTES,
        "date format",
    )?;
    validate_single_line(&settings.selected_color, "theme")?;
    validate_byte_limit(&settings.selected_color, SETTINGS_VALUE_MAX_BYTES, "theme")?;
    validate_single_line(&settings.sort, "sort")?;
    validate_byte_limit(&settings.sort, SETTINGS_VALUE_MAX_BYTES, "sort")?;
    validate_single_line(&settings.preview_position, "preview position")?;
    validate_byte_limit(
        &settings.preview_position,
        SETTINGS_VALUE_MAX_BYTES,
        "preview position",
    )?;
    if let Some(version) = settings.last_run_version.as_deref() {
        validate_single_line(version, "last-run version")?;
        validate_byte_limit(version, SETTINGS_VALUE_MAX_BYTES, "last-run version")?;
    }
    if settings
        .last_update_check_at
        .is_some_and(|timestamp| timestamp < 0)
    {
        return Err(StoreError::Validation(
            "last update check timestamp cannot be negative".into(),
        ));
    }
    if !DATE_FORMATS.contains(&settings.date_format.as_str()) {
        return Err(StoreError::Validation(format!(
            "unknown date format {:?}",
            settings.date_format
        )));
    }
    if !THEMES.contains(&settings.selected_color.as_str()) {
        return Err(StoreError::Validation(format!(
            "unknown theme {:?}",
            settings.selected_color
        )));
    }
    if !SORTS.contains(&settings.sort.as_str()) {
        return Err(StoreError::Validation(format!(
            "unknown sort {:?}",
            settings.sort
        )));
    }
    if !PREVIEW_POSITIONS.contains(&settings.preview_position.as_str()) {
        return Err(StoreError::Validation(format!(
            "unknown preview position {:?}",
            settings.preview_position
        )));
    }
    Ok(())
}

fn read_optional_json<T: DeserializeOwned>(path: &Path) -> Result<Option<T>, StoreError> {
    let file = match fs::File::open(path) {
        Ok(file) => file,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(None),
        Err(error) => return Err(StoreError::io("read", path, error)),
    };
    let size = file
        .metadata()
        .map_err(|error| StoreError::io("inspect", path, error))?
        .len();
    if size > MAX_LEGACY_JSON_BYTES {
        return Err(StoreError::Validation(format!(
            "legacy file {} is larger than the {} MiB safety limit",
            path.display(),
            MAX_LEGACY_JSON_BYTES / 1024 / 1024
        )));
    }
    serde_json::from_reader(std::io::BufReader::new(file))
        .map(Some)
        .map_err(|source| StoreError::Json {
            path: path.to_path_buf(),
            source,
        })
}

fn validate_legacy_schema(path: &Path, schema: Option<u32>) -> Result<(), StoreError> {
    if let Some(found) = schema
        && found != SCHEMA_VERSION
    {
        return Err(StoreError::UnsupportedLegacySchema {
            path: path.to_path_buf(),
            found,
            expected: SCHEMA_VERSION,
        });
    }
    Ok(())
}

#[derive(Debug, Deserialize)]
struct TasksFile {
    schema: u32,
    tasks: Vec<Task>,
}

#[derive(Debug, Deserialize)]
struct CategoriesFile {
    schema: u32,
    categories: Vec<Category>,
}

fn ensure_private_directory(path: &Path) -> Result<(), StoreError> {
    let created = match fs::create_dir(path) {
        Ok(()) => true,
        Err(error) if error.kind() == std::io::ErrorKind::AlreadyExists && path.is_dir() => false,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
            if let Some(parent) = path
                .parent()
                .filter(|parent| !parent.as_os_str().is_empty())
            {
                fs::create_dir_all(parent).map_err(|parent_error| {
                    StoreError::io("create parent directory", parent, parent_error)
                })?;
            }
            match fs::create_dir(path) {
                Ok(()) => true,
                Err(retry)
                    if retry.kind() == std::io::ErrorKind::AlreadyExists && path.is_dir() =>
                {
                    false
                }
                Err(retry) => {
                    return Err(StoreError::io("create directory", path, retry));
                }
            }
        }
        Err(error) => return Err(StoreError::io("create directory", path, error)),
    };
    if created {
        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            fs::set_permissions(path, fs::Permissions::from_mode(0o700))
                .map_err(|error| StoreError::io("set permissions on", path, error))?;
        }
    }
    Ok(())
}

fn prepare_private_database_file(path: &Path) -> Result<(), StoreError> {
    #[cfg(unix)]
    {
        use std::os::unix::fs::OpenOptionsExt;
        match fs::OpenOptions::new()
            .write(true)
            .create_new(true)
            .mode(0o600)
            .open(path)
        {
            Ok(file) => drop(file),
            Err(error) if error.kind() == std::io::ErrorKind::AlreadyExists => {}
            Err(error) => return Err(StoreError::io("create database", path, error)),
        }
    }
    #[cfg(not(unix))]
    let _ = path;
    Ok(())
}

fn set_private_file(path: &Path) -> Result<(), StoreError> {
    #[cfg(unix)]
    {
        use std::os::unix::fs::PermissionsExt;
        fs::set_permissions(path, fs::Permissions::from_mode(0o600))
            .map_err(|error| StoreError::io("set permissions on", path, error))?;
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn default_directory_requires_a_home_when_no_path_is_configured() {
        let error = resolve_data_dir_from(None, None, None)
            .expect_err("missing home must not silently select the working directory");
        assert!(matches!(error, StoreError::Validation(_)));

        assert_eq!(
            resolve_data_dir_from(Some(PathBuf::from("/tmp/mach")), None, None).unwrap(),
            PathBuf::from("/tmp/mach")
        );
        assert_eq!(
            resolve_data_dir_from(None, Some(PathBuf::from("/tmp/configured")), None).unwrap(),
            PathBuf::from("/tmp/configured")
        );
        assert!(resolve_data_dir_from(Some(PathBuf::from("~/.mach")), None, None).is_err());
    }
}