fhir_diff/

diff.rs

//! Changes between two versions of a package, profile by profile and element by element, each
//! classified by what it does to existing data and implementations.
//!
//! **Breaking**: an instance valid against the old profile may be invalid against the new one,
//! or an implementer gains an obligation — a profile removed or replaced, `min` up, `max` down,
//! a type or reference target removed, a new mustSupport, a stronger binding, a new error
//! invariant, a new or changed fixed/pattern value, slicing closed, a required slice added.
//! **Warning**: may break, depending on content the diff does not see — a different value set
//! under a required or extensible binding, a changed invariant expression, a different base, a
//! changed modifier flag, a new warning invariant.
//! **Info**: relaxations and additions that keep old instances valid.

use std::collections::{BTreeMap, BTreeSet};

use serde::Serialize;

use crate::model::{Node, Pool, Profile, Strength, is_variant};

#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize)]
#[serde(rename_all = "lowercase")]
pub enum Severity {
    Info,
    Warning,
    Breaking,
}

impl Severity {
    pub fn as_str(self) -> &'static str {
        match self {
            Self::Info => "info",
            Self::Warning => "warning",
            Self::Breaking => "breaking",
        }
    }
}

/// Every rule, with its severity and what it means.
pub const RULES: &[(&str, Severity, &str)] = &[
    (
        "profile-removed",
        Severity::Breaking,
        "A profile or extension definition was removed",
    ),
    (
        "profile-added",
        Severity::Info,
        "A profile or extension definition was added",
    ),
    (
        "profile-replaced",
        Severity::Breaking,
        "A profile gave way to one with another canonical URL: each is the only one of its type removed or added",
    ),
    (
        "base-changed",
        Severity::Warning,
        "The profile is based on a different definition",
    ),
    (
        "min-increased",
        Severity::Breaking,
        "The minimum cardinality went up",
    ),
    (
        "min-decreased",
        Severity::Info,
        "The minimum cardinality went down",
    ),
    (
        "max-decreased",
        Severity::Breaking,
        "The maximum cardinality went down",
    ),
    (
        "max-increased",
        Severity::Info,
        "The maximum cardinality went up",
    ),
    (
        "type-removed",
        Severity::Breaking,
        "A type the element allowed is no longer allowed",
    ),
    (
        "type-added",
        Severity::Info,
        "The element allows another type",
    ),
    (
        "target-removed",
        Severity::Breaking,
        "A reference target type is no longer allowed",
    ),
    (
        "target-narrowed",
        Severity::Warning,
        "A reference target was narrowed to a profile of the same type",
    ),
    (
        "target-added",
        Severity::Info,
        "A reference may point to another profile",
    ),
    (
        "must-support-added",
        Severity::Breaking,
        "The element became mustSupport",
    ),
    (
        "must-support-removed",
        Severity::Info,
        "The element is no longer mustSupport",
    ),
    (
        "binding-stronger",
        Severity::Breaking,
        "The binding became extensible or required, or required from extensible",
    ),
    (
        "binding-stronger-advisory",
        Severity::Info,
        "The binding strength went up but stays advisory (example → preferred)",
    ),
    (
        "binding-weaker",
        Severity::Info,
        "The binding strength went down",
    ),
    (
        "binding-added",
        Severity::Breaking,
        "A required or extensible binding was added",
    ),
    (
        "value-set-changed",
        Severity::Warning,
        "A required or extensible binding names another value set",
    ),
    (
        "invariant-added",
        Severity::Breaking,
        "An error invariant was added",
    ),
    (
        "invariant-warning-added",
        Severity::Warning,
        "A warning invariant was added",
    ),
    (
        "invariant-changed",
        Severity::Warning,
        "An invariant's expression changed",
    ),
    (
        "invariant-moved",
        Severity::Info,
        "An invariant moved to another element and says the same from there",
    ),
    (
        "invariant-removed",
        Severity::Info,
        "An invariant was removed",
    ),
    (
        "fixed-added",
        Severity::Breaking,
        "A fixed or pattern value was added",
    ),
    (
        "fixed-changed",
        Severity::Breaking,
        "The fixed or pattern value changed",
    ),
    (
        "fixed-removed",
        Severity::Info,
        "The fixed or pattern value was removed",
    ),
    (
        "slicing-closed",
        Severity::Breaking,
        "Slicing became closed",
    ),
    ("slicing-opened", Severity::Info, "Slicing became open"),
    ("slice-added", Severity::Info, "An optional slice was added"),
    ("slice-removed", Severity::Warning, "A slice was removed"),
    (
        "modifier-changed",
        Severity::Warning,
        "The isModifier flag changed",
    ),
];

pub fn severity_of(rule: &str) -> Severity {
    RULES
        .iter()
        .find(|(id, _, _)| *id == rule)
        .map_or(Severity::Info, |(_, severity, _)| *severity)
}

/// One change.
#[derive(Debug, Clone, PartialEq, Serialize)]
pub struct Change {
    pub rule: &'static str,
    pub severity: Severity,
    /// Element path, slices written `:name` (`Patient.identifier:mrn.system`).
    pub path: String,
    pub message: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub old: Option<String>,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub new: Option<String>,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
#[serde(rename_all = "lowercase")]
pub enum Status {
    Added,
    Removed,
    Changed,
    Unchanged,
}

/// The changes of one profile.
#[derive(Debug, Clone, Serialize)]
pub struct ProfileDiff {
    pub url: String,
    pub name: String,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub title: Option<String>,
    pub type_name: String,
    pub status: Status,
    /// Canonical URL of the profile this one took over from.
    #[serde(skip_serializing_if = "Option::is_none")]
    pub replaces: Option<String>,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub old_version: Option<String>,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub new_version: Option<String>,
    /// File in the new package (the old one for a removed profile).
    pub file: String,
    pub changes: Vec<Change>,
}

impl ProfileDiff {
    pub fn worst(&self) -> Option<Severity> {
        self.changes.iter().map(|c| c.severity).max()
    }
}

/// The diff of two packages.
#[derive(Debug, Clone, Serialize)]
pub struct Report {
    pub old: String,
    pub new: String,
    pub profiles: Vec<ProfileDiff>,
    /// Definitions that could not be read, and bases that were missing or taken from the other
    /// package.
    #[serde(skip_serializing_if = "Vec::is_empty")]
    pub notes: Vec<String>,
}

impl Report {
    pub fn count(&self, severity: Severity) -> usize {
        self.profiles
            .iter()
            .flat_map(|p| &p.changes)
            .filter(|c| c.severity == severity)
            .count()
    }

    pub fn worst(&self) -> Option<Severity> {
        self.profiles.iter().filter_map(ProfileDiff::worst).max()
    }
}

/// Which profiles to compare.
#[derive(Debug, Clone, Default)]
pub struct Filter {
    /// URLs or names; empty compares all.
    pub profiles: Vec<String>,
}

impl Filter {
    fn admits(&self, profile: &Profile) -> bool {
        self.profiles.is_empty()
            || self
                .profiles
                .iter()
                .any(|p| p == &profile.url || p == &profile.name)
    }
}

/// Compare the profiles of two pools.
pub fn diff(old_id: &str, old: &Pool, new_id: &str, new: &Pool, filter: &Filter) -> Report {
    let (old_all, new_all) = (old.profiles(), new.profiles());
    // A profile is chosen on either side: renamed between versions (US Core `USCoreLocation` →
    // `USCoreLocationProfile`), its canonical URL still pairs the two.
    let chosen: BTreeSet<String> = old_all
        .iter()
        .chain(&new_all)
        .filter(|p| filter.admits(p))
        .map(|p| p.url.clone())
        .collect();
    let keep = |profiles: Vec<Profile>| -> BTreeMap<String, Profile> {
        profiles
            .into_iter()
            .filter(|p| chosen.contains(&p.url))
            .map(|p| (p.url.clone(), p))
            .collect()
    };
    let (old_profiles, new_profiles) = (keep(old_all), keep(new_all));
    let urls: BTreeSet<&String> = old_profiles.keys().chain(new_profiles.keys()).collect();
    let replacements = replacements(&old_profiles, &new_profiles);
    let replaced: BTreeSet<&String> = replacements.values().collect();
    let mut profiles = Vec::new();
    for url in urls {
        if replaced.contains(url) {
            continue;
        }
        let predecessor = replacements
            .get(url)
            .and_then(|previous| old_profiles.get(previous));
        let entry = match (predecessor.or(old_profiles.get(url)), new_profiles.get(url)) {
            (Some(o), Some(n)) => {
                let mut changes = Vec::new();
                if o.url != n.url {
                    changes.push(change(
                        "profile-replaced",
                        &n.type_name,
                        &format!("Takes over from {}", o.url),
                        Some(o.url.clone()),
                        Some(n.url.clone()),
                    ));
                }
                if o.base != n.base {
                    changes.push(change(
                        "base-changed",
                        &n.type_name,
                        "The profile is based on another definition",
                        o.base.clone(),
                        n.base.clone(),
                    ));
                }
                let mut walker = Walker {
                    old,
                    new,
                    changes: &mut changes,
                    invariants_added: Vec::new(),
                    invariants_removed: Vec::new(),
                };
                walker.node(&n.type_name, &n.type_name, &o.tree, &n.tree);
                let (added, removed) = (walker.invariants_added, walker.invariants_removed);
                fold_moved_invariants(&mut changes, &added, &removed);
                ProfileDiff {
                    url: url.clone(),
                    name: n.name.clone(),
                    title: n.title.clone(),
                    type_name: n.type_name.clone(),
                    status: if changes.is_empty() {
                        Status::Unchanged
                    } else {
                        Status::Changed
                    },
                    replaces: (o.url != n.url).then(|| o.url.clone()),
                    old_version: o.version.clone(),
                    new_version: n.version.clone(),
                    file: n.file.clone(),
                    changes,
                }
            }
            (Some(o), None) => ProfileDiff {
                url: url.clone(),
                name: o.name.clone(),
                title: o.title.clone(),
                type_name: o.type_name.clone(),
                status: Status::Removed,
                replaces: None,
                old_version: o.version.clone(),
                new_version: None,
                file: o.file.clone(),
                changes: vec![change(
                    "profile-removed",
                    &o.type_name,
                    "The profile is not in the new package",
                    Some(url.clone()),
                    None,
                )],
            },
            (None, Some(n)) => ProfileDiff {
                url: url.clone(),
                name: n.name.clone(),
                title: n.title.clone(),
                type_name: n.type_name.clone(),
                status: Status::Added,
                replaces: None,
                old_version: None,
                new_version: n.version.clone(),
                file: n.file.clone(),
                changes: vec![change(
                    "profile-added",
                    &n.type_name,
                    "The profile is new",
                    None,
                    Some(url.clone()),
                )],
            },
            (None, None) => continue,
        };
        profiles.push(entry);
    }
    let notes = old
        .skipped
        .iter()
        .map(|(file, reason)| format!("{old_id}: {file}: {reason}"))
        .chain(
            new.skipped
                .iter()
                .map(|(file, reason)| format!("{new_id}: {file}: {reason}")),
        )
        .chain(old.missing.borrow().iter().map(|base| {
            format!("{old_id}: base {base} is in no loaded package; compared over the core type")
        }))
        .chain(new.missing.borrow().iter().map(|base| {
            format!("{new_id}: base {base} is in no loaded package; compared over the core type")
        }))
        .chain(old.borrowed.iter().map(|base| {
            format!("{old_id}: base {base} is in no listed dependency; taken from {new_id}'s")
        }))
        .chain(new.borrowed.iter().map(|base| {
            format!("{new_id}: base {base} is in no listed dependency; taken from {old_id}'s")
        }))
        .collect();
    Report {
        old: old_id.to_owned(),
        new: new_id.to_owned(),
        profiles,
        notes,
    }
}

/// Profiles that took over from another under a new canonical URL, new URL to old. A package
/// says nothing about this, so the pair is inferred, and only where it leaves no choice: the one
/// profile of a type removed and the one added. Extensions are left out, since two of them on
/// the same type say nothing about each other.
fn replacements(
    old: &BTreeMap<String, Profile>,
    new: &BTreeMap<String, Profile>,
) -> BTreeMap<String, String> {
    let only_here = |side: &'_ BTreeMap<String, Profile>, other: &BTreeMap<String, Profile>| {
        let mut by_type: BTreeMap<String, Vec<String>> = BTreeMap::new();
        for profile in side.values() {
            if !other.contains_key(&profile.url) && profile.type_name != "Extension" {
                by_type
                    .entry(profile.type_name.clone())
                    .or_default()
                    .push(profile.url.clone());
            }
        }
        by_type
    };
    let (removed, added) = (only_here(old, new), only_here(new, old));
    removed
        .iter()
        .filter_map(
            |(type_name, urls)| match (urls.as_slice(), added.get(type_name)?.as_slice()) {
                ([previous], [next]) => Some((next.clone(), previous.clone())),
                _ => None,
            },
        )
        .collect()
}

/// An invariant found on one element and gone from another.
struct InvariantSite {
    key: String,
    severity: String,
    /// Its change in the profile's list.
    index: usize,
}

/// An invariant that left one element and arrived on another is one change, not a removal and
/// an addition.
fn fold_moved_invariants(
    changes: &mut Vec<Change>,
    added: &[InvariantSite],
    removed: &[InvariantSite],
) {
    let mut folded = BTreeSet::new();
    for site in added {
        let Some(from) = removed
            .iter()
            .find(|r| r.key == site.key && !folded.contains(&r.index))
        else {
            continue;
        };
        let origin = changes[from.index].clone();
        let (old_expression, arrival) = (origin.old.unwrap_or_default(), &mut changes[site.index]);
        let new_expression = arrival.new.clone().unwrap_or_default();
        let same = from.severity == site.severity
            && same_after_move(
                &origin.path,
                &old_expression,
                &arrival.path,
                &new_expression,
            );
        let (rule, message) = if same {
            (
                "invariant-moved",
                format!("Invariant {} moved from {}", site.key, origin.path),
            )
        } else {
            (
                "invariant-changed",
                format!(
                    "Invariant {} moved from {} and changed",
                    site.key, origin.path
                ),
            )
        };
        arrival.rule = rule;
        arrival.severity = severity_of(rule);
        arrival.message = message;
        arrival.old = Some(old_expression);
        folded.insert(from.index);
    }
    let mut index = 0;
    changes.retain(|_| {
        index += 1;
        !folded.contains(&(index - 1))
    });
}

/// Whether two expressions say the same from their own elements: `value.ofType(Quantity)` on
/// `Observation` is `ofType(Quantity)` on `Observation.value`.
fn same_after_move(from: &str, old: &str, to: &str, new: &str) -> bool {
    match (to.strip_prefix(from), from.strip_prefix(to)) {
        (Some(tail), _) if tail.starts_with('.') => relative_to(old, &tail[1..]) == new,
        (_, Some(tail)) if tail.starts_with('.') => relative_to(new, &tail[1..]) == old,
        _ => old == new,
    }
}

/// `expression` with the leading `element.` taken off every path that starts with it.
fn relative_to(expression: &str, element: &str) -> String {
    let prefix = format!("{element}.");
    let mut result = String::new();
    let mut rest = expression;
    while let Some(at) = rest.find(&prefix) {
        result.push_str(&rest[..at]);
        let inside_a_path = result
            .chars()
            .next_back()
            .is_some_and(|c| c.is_alphanumeric() || c == '.' || c == '_');
        if inside_a_path {
            result.push_str(&prefix);
        }
        rest = &rest[at + prefix.len()..];
    }
    result.push_str(rest);
    result
}

fn change(
    rule: &'static str,
    path: &str,
    message: &str,
    old: Option<String>,
    new: Option<String>,
) -> Change {
    Change {
        rule,
        severity: severity_of(rule),
        path: path.to_owned(),
        message: message.to_owned(),
        old,
        new,
    }
}

fn cardinality(max: Option<u32>) -> String {
    max.map_or_else(|| "*".to_owned(), |m| m.to_string())
}

struct Walker<'a> {
    old: &'a Pool,
    new: &'a Pool,
    changes: &'a mut Vec<Change>,
    invariants_added: Vec<InvariantSite>,
    invariants_removed: Vec<InvariantSite>,
}

impl Walker<'_> {
    fn push(
        &mut self,
        rule: &'static str,
        path: &str,
        message: String,
        old: Option<String>,
        new: Option<String>,
    ) {
        self.changes.push(Change {
            rule,
            severity: severity_of(rule),
            path: path.to_owned(),
            message,
            old,
            new,
        });
    }

    /// Compare two versions of one element; `type_name` is the element's own type, for
    /// looking up children only one version constrains.
    fn node(&mut self, path: &str, type_name: &str, old: &Node, new: &Node) {
        self.element(path, type_name, old, new, false, &BTreeMap::new());
    }

    /// Compare an element; a choice `variant` (`valueQuantity`) repeats what its choice
    /// element says about cardinality, flags, invariants and type slices, so only its own
    /// binding, value and children are compared. `old_siblings` are the elements next to `old`,
    /// where a choice keeps its variants.
    fn element(
        &mut self,
        path: &str,
        type_name: &str,
        old: &Node,
        new: &Node,
        variant: bool,
        old_siblings: &BTreeMap<String, Node>,
    ) {
        if !variant {
            self.cardinality(path, old, new);
            self.types(path, old, new);
            self.flags(path, old, new);
            self.constraints(path, old, new);
            self.slicing(path, old, new);
        }
        self.binding(path, old, new);
        self.pattern(path, old, new);

        let names: BTreeSet<&String> = old.children.keys().chain(new.children.keys()).collect();
        for name in names {
            let child_path = format!("{path}.{name}");
            let (o, n) = (old.children.get(name), new.children.get(name));
            let child_type = n
                .or(o)
                .and_then(|c| (c.types.len() == 1).then(|| c.types.iter().next().cloned()))
                .flatten()
                .unwrap_or_default();
            let variant = is_variant(&new.children, name) || is_variant(&old.children, name);
            match (o, n) {
                (Some(o), Some(n)) => {
                    if variant && choice_settled(old, new, name) {
                        self.flags(&child_path, o, n);
                    }
                    self.element(&child_path, &child_type, o, n, variant, &old.children);
                }
                (Some(o), None) => {
                    let base = self.base(type_name, name, o);
                    self.element(&child_path, &child_type, o, &base, variant, &old.children);
                }
                (None, Some(n)) => {
                    let base = self.base(type_name, name, n);
                    self.element(&child_path, &child_type, &base, n, variant, &old.children);
                }
                (None, None) => {}
            }
        }

        if variant {
            return;
        }
        let slices: BTreeSet<&String> = old.slices.keys().chain(new.slices.keys()).collect();
        for slice in slices {
            let slice_path = format!("{path}:{slice}");
            match (old.slices.get(slice), new.slices.get(slice)) {
                (Some(o), Some(n)) => self.node(&slice_path, type_name, o, n),
                (Some(_), None) => self.push(
                    "slice-removed",
                    &slice_path,
                    format!("Slice {slice} was removed"),
                    Some(slice.clone()),
                    None,
                ),
                (None, Some(n)) => {
                    if n.min > 0 {
                        self.push(
                            "min-increased",
                            &slice_path,
                            format!("Required slice {slice} was added (min {})", n.min),
                            Some("0".into()),
                            Some(n.min.to_string()),
                        );
                    } else {
                        self.push(
                            "slice-added",
                            &slice_path,
                            format!("Optional slice {slice} was added"),
                            None,
                            Some(slice.clone()),
                        );
                    }
                    // A type slice of a choice asks for nothing new when that type already was
                    // mustSupport.
                    let supported = old_siblings.get(slice).is_some_and(|v| v.must_support);
                    if n.must_support && !supported {
                        self.push(
                            "must-support-added",
                            &slice_path,
                            format!("New slice {slice} is mustSupport"),
                            None,
                            Some("true".into()),
                        );
                    }
                }
                (None, None) => {}
            }
        }
    }

    /// What an element only one version constrains is in the other: its core definition,
    /// else the same element without constraints.
    fn base(&self, type_name: &str, name: &str, present: &Node) -> Node {
        let core = self
            .new
            .base_node(type_name, name)
            .or_else(|| self.old.base_node(type_name, name));
        match core {
            Some(mut base) => {
                // The core definition has no nested constraints of its own here.
                base.children.clear();
                base
            }
            // Not a core element (a backbone child, a logical model): compare the
            // constraints alone, keeping the element's shape.
            None => Node {
                types: present.types.clone(),
                max: present.max,
                targets: present.targets.clone(),
                ..Node::unconstrained()
            },
        }
    }

    fn cardinality(&mut self, path: &str, old: &Node, new: &Node) {
        if new.min > old.min {
            self.push(
                "min-increased",
                path,
                format!("min {} → {}", old.min, new.min),
                Some(old.min.to_string()),
                Some(new.min.to_string()),
            );
        } else if new.min < old.min {
            self.push(
                "min-decreased",
                path,
                format!("min {} → {}", old.min, new.min),
                Some(old.min.to_string()),
                Some(new.min.to_string()),
            );
        }
        let narrower = match (old.max, new.max) {
            (None, Some(_)) => Some(true),
            (Some(_), None) => Some(false),
            (Some(a), Some(b)) if b < a => Some(true),
            (Some(a), Some(b)) if b > a => Some(false),
            _ => None,
        };
        if let Some(narrower) = narrower {
            self.push(
                if narrower {
                    "max-decreased"
                } else {
                    "max-increased"
                },
                path,
                format!("max {} → {}", cardinality(old.max), cardinality(new.max)),
                Some(cardinality(old.max)),
                Some(cardinality(new.max)),
            );
        }
    }

    fn types(&mut self, path: &str, old: &Node, new: &Node) {
        // A node that names no type allows its base's types.
        if !old.types.is_empty() && !new.types.is_empty() {
            for removed in old.types.difference(&new.types) {
                self.push(
                    "type-removed",
                    path,
                    format!("Type {removed} is no longer allowed"),
                    Some(removed.clone()),
                    None,
                );
            }
            for added in new.types.difference(&old.types) {
                self.push(
                    "type-added",
                    path,
                    format!("Type {added} is allowed"),
                    None,
                    Some(added.clone()),
                );
            }
        }
        if old.targets.is_empty() && !new.targets.is_empty() {
            self.push(
                "target-removed",
                path,
                format!("References are restricted to {}", join(&new.targets)),
                None,
                Some(join(&new.targets)),
            );
        } else if !new.targets.is_empty() {
            let mut added: Vec<&String> = new.targets.difference(&old.targets).collect();
            for removed in old.targets.difference(&new.targets) {
                let removed_type = self.old.type_of(removed);
                // `Patient` → `us-core-patient`: the same type, now with a profile.
                let narrowed = added
                    .iter()
                    .position(|a| removed_type.is_some() && self.new.type_of(a) == removed_type);
                match narrowed {
                    Some(index) => {
                        let to = added.remove(index);
                        self.push(
                            "target-narrowed",
                            path,
                            format!("Target {removed} narrowed to {to}"),
                            Some(removed.clone()),
                            Some(to.clone()),
                        );
                    }
                    None => self.push(
                        "target-removed",
                        path,
                        format!("Target {removed} is no longer allowed"),
                        Some(removed.clone()),
                        None,
                    ),
                }
            }
            for added in added {
                self.push(
                    "target-added",
                    path,
                    format!("Target {added} is allowed"),
                    None,
                    Some(added.clone()),
                );
            }
        }
    }

    fn flags(&mut self, path: &str, old: &Node, new: &Node) {
        match (old.must_support, new.must_support) {
            (false, true) => self.push(
                "must-support-added",
                path,
                "Became mustSupport".into(),
                Some("false".into()),
                Some("true".into()),
            ),
            (true, false) => self.push(
                "must-support-removed",
                path,
                "No longer mustSupport".into(),
                Some("true".into()),
                Some("false".into()),
            ),
            _ => {}
        }
        // Profiles cannot change isModifier on an element; an extension definition can
        // (its root): compare it there only.
        if !path.contains('.') && old.is_modifier != new.is_modifier {
            self.push(
                "modifier-changed",
                path,
                format!("isModifier {} → {}", old.is_modifier, new.is_modifier),
                Some(old.is_modifier.to_string()),
                Some(new.is_modifier.to_string()),
            );
        }
    }

    fn binding(&mut self, path: &str, old: &Node, new: &Node) {
        let describe = |b: &crate::model::Binding| {
            format!(
                "{} {}",
                b.strength.as_str(),
                b.value_set.as_deref().unwrap_or("(no value set)")
            )
        };
        match (&old.binding, &new.binding) {
            (None, Some(n)) if n.strength >= Strength::Extensible => {
                self.push(
                    "binding-added",
                    path,
                    format!("Bound {}", describe(n)),
                    None,
                    Some(describe(n)),
                );
            }
            (Some(o), Some(n)) => {
                if n.strength > o.strength {
                    // Only an extensible or required binding can make existing data invalid.
                    let rule = if n.strength >= Strength::Extensible {
                        "binding-stronger"
                    } else {
                        "binding-stronger-advisory"
                    };
                    self.push(
                        rule,
                        path,
                        format!("{} → {}", o.strength.as_str(), n.strength.as_str()),
                        Some(describe(o)),
                        Some(describe(n)),
                    );
                } else if n.strength < o.strength {
                    self.push(
                        "binding-weaker",
                        path,
                        format!("{} → {}", o.strength.as_str(), n.strength.as_str()),
                        Some(describe(o)),
                        Some(describe(n)),
                    );
                }
                let plain = |vs: &Option<String>| {
                    vs.as_deref()
                        .map(|v| v.split('|').next().unwrap_or(v).to_owned())
                };
                if plain(&o.value_set) != plain(&n.value_set) && n.strength >= Strength::Extensible
                {
                    self.push(
                        "value-set-changed",
                        path,
                        format!(
                            "Value set {} → {}",
                            o.value_set.as_deref().unwrap_or("-"),
                            n.value_set.as_deref().unwrap_or("-")
                        ),
                        o.value_set.clone(),
                        n.value_set.clone(),
                    );
                }
            }
            _ => {}
        }
    }

    fn constraints(&mut self, path: &str, old: &Node, new: &Node) {
        for (key, constraint) in &new.constraints {
            match old.constraints.get(key) {
                None => {
                    let rule = if constraint.severity == "error" {
                        "invariant-added"
                    } else {
                        "invariant-warning-added"
                    };
                    self.invariants_added.push(InvariantSite {
                        key: key.clone(),
                        severity: constraint.severity.clone(),
                        index: self.changes.len(),
                    });
                    self.push(
                        rule,
                        path,
                        format!("Invariant {key}: {}", constraint.human),
                        None,
                        Some(constraint.expression.clone()),
                    );
                }
                Some(previous) if previous.expression != constraint.expression => {
                    self.push(
                        "invariant-changed",
                        path,
                        format!("Invariant {key} changed"),
                        Some(previous.expression.clone()),
                        Some(constraint.expression.clone()),
                    );
                }
                Some(_) => {}
            }
        }
        for (key, constraint) in &old.constraints {
            if !new.constraints.contains_key(key) {
                self.invariants_removed.push(InvariantSite {
                    key: key.clone(),
                    severity: constraint.severity.clone(),
                    index: self.changes.len(),
                });
                self.push(
                    "invariant-removed",
                    path,
                    format!("Invariant {key} was removed"),
                    Some(constraint.expression.clone()),
                    None,
                );
            }
        }
    }

    fn pattern(&mut self, path: &str, old: &Node, new: &Node) {
        let text = |v: &serde_json::Value| v.to_string();
        match (&old.pattern, &new.pattern) {
            (None, Some(n)) => self.push(
                "fixed-added",
                path,
                format!("Fixed/pattern {}", text(n)),
                None,
                Some(text(n)),
            ),
            (Some(o), None) => self.push(
                "fixed-removed",
                path,
                "Fixed/pattern value removed".into(),
                Some(text(o)),
                None,
            ),
            (Some(o), Some(n)) if o != n => self.push(
                "fixed-changed",
                path,
                "Fixed/pattern value changed".into(),
                Some(text(o)),
                Some(text(n)),
            ),
            _ => {}
        }
    }

    fn slicing(&mut self, path: &str, old: &Node, new: &Node) {
        let rules = |node: &Node| node.slicing.as_ref().map(|s| s.rules.clone());
        match (rules(old).as_deref(), rules(new).as_deref()) {
            (Some("closed"), Some(r)) if r != "closed" => self.push(
                "slicing-opened",
                path,
                format!("Slicing closed → {r}"),
                Some("closed".into()),
                Some(r.to_owned()),
            ),
            (o, Some("closed")) if o != Some("closed") => self.push(
                "slicing-closed",
                path,
                format!("Slicing {} → closed", o.unwrap_or("none")),
                o.map(str::to_owned),
                Some("closed".into()),
            ),
            _ => {}
        }
    }
}

/// A variant's own mustSupport means something only while its choice stays put: a choice whose
/// flag changed is reported itself, and one sliced by type speaks through its slices.
fn choice_settled(old: &Node, new: &Node, variant: &str) -> bool {
    let choice = |parent: &'_ Node| {
        parent
            .children
            .values()
            .find(|c| c.types.contains(variant))
            .map(|c| (c.must_support, c.slices.is_empty()))
    };
    matches!(
        (choice(old), choice(new)),
        (Some((before, true)), Some((after, true))) if before == after
    )
}

fn join(set: &BTreeSet<String>) -> String {
    set.iter().cloned().collect::<Vec<_>>().join(", ")
}

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

    #[test]
    fn an_expression_reads_the_same_from_a_child_element() {
        let old = "value.ofType(Quantity).system.empty() or value.ofType(Quantity).system = 'u'";
        let new = "ofType(Quantity).system.empty() or ofType(Quantity).system = 'u'";
        assert!(same_after_move(
            "Observation",
            old,
            "Observation.value",
            new
        ));
        assert!(same_after_move(
            "Observation.value",
            new,
            "Observation",
            old
        ));
        // The same text on another element means something else.
        assert!(!same_after_move(
            "Observation",
            old,
            "Observation.value",
            old
        ));
    }

    #[test]
    fn only_paths_that_start_with_the_element_are_rewritten() {
        assert_eq!(
            relative_to("a.value.b or value.c", "value"),
            "a.value.b or c"
        );
        assert_eq!(relative_to("myvalue.x", "value"), "myvalue.x");
    }
}