soldb_debugger/

lib.rs

//! Source-level debugging over an execution trace.
//!
//! Given a [`soldb_core::TransactionTrace`] and the ETHDebug metadata for the contract
//! it executed, this crate answers the questions a source-level debugger asks: which
//! source span a step is at, which function contains a program counter, and which
//! variables are live there with what values.
//!
//! It is frontend-agnostic on purpose. [`variables_for_step`] is what backs both the CLI
//! REPL's `vars`/`print` commands and the DAP server's variables view, so the terminal
//! and the editor decode identically.
//!
//! Variable values are only as good as the debug info: a location the backend did not
//! capture decodes to [`DebugValueStatus::Unavailable`], and a value whose declared type
//! is not decodable is reported as [`DebugValueStatus::Raw`] rather than guessed at.

use std::collections::BTreeMap;

use serde::{Deserialize, Serialize};

use soldb_core::{StepSnapshot, TraceStep, TransactionTrace, Word as StackWord};
use soldb_ethdebug::{decode_value, parse_word, EthdebugInfo, VariableLocation, Word};

use crate::decode::ValueReader;

pub mod condition;
pub mod debug_diff;
mod decode;
mod locals;
pub mod state;
pub mod stepping;
mod types;

pub use condition::{Condition, ConditionContext, Evaluation};
pub use debug_diff::{
    capture_debug_trace, compare_debug_traces, DebugCheckpoint, DebugDiffMode, DebugDiffReport,
    DebugDifference, DebugDifferenceKind, DebugExecution, DebugTrace, DebugTraceEvent,
    DebugTraceSummary,
};
pub use soldb_ethdebug::{CodeGenerator, StorageLayout};
pub use state::{
    short_hex, state_value, state_variables, CachedChain, ChainRead, ChainStorage, StateSource,
    StateVariable, StorageTape, StorageWords,
};
pub use stepping::{
    address_from_word, call_target, normalize_address, source_path_matches, ContractDebugInfo,
    Frame, FunctionId, InferredVariable, JumpMarker, LineKey, LocalsStatus, ResolvedFunction,
    ResolvedLine, SourceListing, StepLocation, StepMap, VariableKind, INFERRED_LOCALS_NOTE,
    INFERRED_LOCALS_WARNING,
};
pub use types::{SourceTypes, StructMember};

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct DebugSession {
    pub trace: TransactionTrace,
    pub ethdebug: Option<EthdebugInfo>,
    #[serde(default)]
    pub source_contents: BTreeMap<u64, String>,
    #[serde(default)]
    pub functions: Vec<SourceFunction>,
}

impl DebugSession {
    #[must_use]
    pub fn new(trace: TransactionTrace) -> Self {
        Self {
            trace,
            ethdebug: None,
            source_contents: BTreeMap::new(),
            functions: Vec::new(),
        }
    }

    #[must_use]
    pub fn with_ethdebug(
        trace: TransactionTrace,
        ethdebug: EthdebugInfo,
        source_contents: BTreeMap<u64, String>,
    ) -> Self {
        let mut session = Self::new(trace);
        session.attach_ethdebug(ethdebug, source_contents);
        session
    }

    /// Attaches debug info to a session that already holds a trace, replacing whatever
    /// was attached before.
    ///
    /// The trace is left in place rather than rebuilt, so a frontend that keeps a large
    /// trace resident can add or swap the contract's artifacts without copying hundreds
    /// of thousands of steps.
    pub fn attach_ethdebug(
        &mut self,
        ethdebug: EthdebugInfo,
        source_contents: BTreeMap<u64, String>,
    ) {
        self.functions = source_contents
            .iter()
            .flat_map(|(source_id, source)| parse_source_functions(*source_id, source))
            .collect();
        self.ethdebug = Some(ethdebug);
        self.source_contents = source_contents;
    }

    #[must_use]
    pub fn step(&self, step_index: usize) -> Option<DebugStep> {
        let step = self.trace.steps.get(step_index)?;
        Some(DebugStep {
            index: step_index,
            pc: step.pc,
            op: step.op.to_string(),
            gas: step.gas,
            gas_cost: step.gas_cost,
            depth: step.depth,
            source: self.source_span(step.pc),
            function: self.function_at_pc(step.pc).cloned(),
            snapshot: step.normalized_snapshot(),
            variables: self.variables_at_step(step),
        })
    }

    #[must_use]
    pub fn steps(&self) -> Vec<DebugStep> {
        (0..self.trace.steps.len())
            .filter_map(|step_index| self.step(step_index))
            .collect()
    }

    #[must_use]
    pub fn source_span(&self, pc: u64) -> Option<SourceSpan> {
        let ethdebug = self.ethdebug.as_ref()?;
        let instruction = ethdebug.instruction_at_pc(pc)?;
        let location = instruction.source_location()?;
        let path = ethdebug.sources.get(&location.source_id)?.clone();
        let position = self
            .source_contents
            .get(&location.source_id)
            .map(|source| line_column_for_offset(source, location.offset as usize));
        Some(SourceSpan {
            source_id: location.source_id,
            path,
            offset: location.offset,
            length: location.length,
            line: position.map_or(0, |position| position.line),
            column: position.map_or(0, |position| position.column),
        })
    }

    #[must_use]
    pub fn function_at_pc(&self, pc: u64) -> Option<&SourceFunction> {
        let location = self
            .ethdebug
            .as_ref()?
            .instruction_at_pc(pc)?
            .source_location()?;
        self.functions
            .iter()
            .filter(|function| {
                function.source_id == location.source_id
                    && function.declaration_start <= location.offset
                    && location.offset <= function.body_end
            })
            .min_by_key(|function| function.body_end.saturating_sub(function.declaration_start))
    }

    #[must_use]
    pub fn variables_at_step(&self, step: &TraceStep) -> Vec<DebugVariable> {
        let Some(ethdebug) = &self.ethdebug else {
            return Vec::new();
        };
        variables_for_step(&self.trace, ethdebug, step)
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct DebugStep {
    pub index: usize,
    pub pc: u64,
    pub op: String,
    pub gas: u64,
    pub gas_cost: u64,
    pub depth: u64,
    pub source: Option<SourceSpan>,
    pub function: Option<SourceFunction>,
    pub snapshot: StepSnapshot,
    pub variables: Vec<DebugVariable>,
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct SourceSpan {
    pub source_id: u64,
    pub path: String,
    pub offset: u64,
    pub length: u64,
    pub line: u64,
    pub column: u64,
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct SourceFunction {
    pub source_id: u64,
    /// The declared name; `constructor`, `fallback`, and `receive` for those, and the
    /// modifier's name for a modifier.
    pub name: String,
    pub params: Vec<SourceParam>,
    pub declaration_start: u64,
    /// The one-based line the declaration begins on.
    #[serde(default)]
    pub declaration_line: u64,
    /// The offset of the body's opening brace.
    #[serde(default)]
    pub body_start: u64,
    pub body_end: u64,
    /// The return parameters, named or not, in declaration order.
    #[serde(default)]
    pub returns: Vec<SourceLocal>,
    /// The local variables declared in the body, in declaration order.
    #[serde(default)]
    pub locals: Vec<SourceLocal>,
    /// Whether the header invokes modifiers, which run inlined around the body and reserve
    /// stack slots of their own.
    #[serde(default)]
    pub has_modifiers: bool,
    /// The modifiers the header invokes, in the order they run.
    #[serde(default)]
    pub modifiers: Vec<String>,
    /// For a modifier, the offset of its `_;` placeholder, where the function body runs.
    #[serde(default)]
    pub placeholder: Option<u64>,
}

/// A half-open byte range in a source.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct ByteRange {
    pub start: u64,
    pub end: u64,
}

impl ByteRange {
    #[must_use]
    pub const fn contains(self, offset: u64) -> bool {
        self.start <= offset && offset < self.end
    }

    /// Whether a span of `length` bytes at `offset` lies inside this range.
    #[must_use]
    pub const fn covers(self, offset: u64, length: u64) -> bool {
        self.start <= offset && offset.saturating_add(length) <= self.end
    }

    /// Whether this range contains the whole of `other`.
    #[must_use]
    pub const fn encloses(self, other: Self) -> bool {
        self.start <= other.start && other.end <= self.end
    }
}

/// A variable a function declares in its source: a return parameter or a local.
///
/// The spans are what ties the variable to the trace. solc's legacy code generator reserves
/// the variable's stack slot under `declaration`, the `type [location] name` text, as the
/// first instructions of `statement`; the slot lives until execution leaves `scope`.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct SourceLocal {
    pub name: String,
    /// The type without its data location, such as `uint256` or `string`.
    pub ty: String,
    /// The data location as declared, when the type has one.
    #[serde(default)]
    pub location: Option<String>,
    pub declaration: ByteRange,
    /// The declaration statement, from its first token to before its `;`.
    pub statement: ByteRange,
    /// The variable's index among those its statement declares: each takes the next slot.
    pub position: usize,
    pub scope: ByteRange,
}

impl SourceLocal {
    /// The type as declared, with its data location.
    #[must_use]
    pub fn declared_type(&self) -> String {
        match &self.location {
            Some(location) => format!("{} {location}", self.ty),
            None => self.ty.clone(),
        }
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct SourceParam {
    pub name: String,
    /// The type without its data location, such as `uint256` or `string`.
    pub ty: String,
    /// The data location as declared: `memory`, `calldata`, or `storage`; `None` for a
    /// value type.
    #[serde(default)]
    pub location: Option<String>,
}

/// One argument of a function frame, as it was on the stack when the frame was entered.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct FrameArgument {
    pub name: String,
    pub ty: String,
    pub value: DebugValue,
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct DebugVariable {
    pub name: String,
    pub ty: String,
    pub location: DebugLocation,
    pub value: DebugValue,
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct DebugLocation {
    pub kind: String,
    pub offset: u64,
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
pub struct DebugValue {
    pub display: String,
    pub raw: Option<String>,
    pub status: DebugValueStatus,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum DebugValueStatus {
    Decoded,
    Raw,
    Unavailable,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct SourcePosition {
    line: u64,
    column: u64,
}

#[must_use]
pub fn variables_for_step(
    trace: &TransactionTrace,
    ethdebug: &EthdebugInfo,
    step: &TraceStep,
) -> Vec<DebugVariable> {
    ethdebug
        .variables_at_pc(step.pc)
        .into_iter()
        .map(|variable| decode_variable(trace, step, variable))
        .collect()
}

fn decode_variable(
    trace: &TransactionTrace,
    step: &TraceStep,
    variable: &VariableLocation,
) -> DebugVariable {
    let raw = raw_value_for_location(trace, step, variable);
    let value = raw.map_or_else(
        || DebugValue {
            display: "<unavailable>".to_owned(),
            raw: None,
            status: DebugValueStatus::Unavailable,
        },
        |raw| decode_debug_value(&raw, &variable.ty),
    );
    DebugVariable {
        name: variable.name.clone(),
        ty: variable.ty.clone(),
        location: DebugLocation {
            kind: variable.location_type.clone(),
            offset: variable.offset,
        },
        value,
    }
}

fn raw_value_for_location(
    trace: &TransactionTrace,
    step: &TraceStep,
    variable: &VariableLocation,
) -> Option<String> {
    let snapshot = step.snapshot_ref();
    match variable.location_type.as_str() {
        "stack" => snapshot
            .stack
            .get(variable.offset as usize)
            .map(|value| normalize_hex(value)),
        "memory" => word_from_hex_bytes(snapshot.memory?, variable.offset as usize),
        "calldata" => word_from_hex_bytes(&trace.input_data, variable.offset as usize),
        "storage" => storage_value(snapshot.storage, variable.offset),
        _ => None,
    }
}

pub(crate) fn decode_debug_value(raw: &str, ty: &str) -> DebugValue {
    let ty = ty.trim();
    let normalized = normalize_hex(raw);
    let word = normalized.trim_start_matches("0x");
    let decoded = decode_static_word(word, ty);
    match decoded {
        Some(display) => DebugValue {
            display,
            raw: Some(normalized),
            status: DebugValueStatus::Decoded,
        },
        None => DebugValue {
            display: normalized.clone(),
            raw: Some(normalized),
            status: DebugValueStatus::Raw,
        },
    }
}

pub(crate) fn decode_static_word(word: &str, ty: &str) -> Option<String> {
    if !word.bytes().all(|byte| byte.is_ascii_hexdigit()) {
        return None;
    }
    if ty.starts_with("uint") {
        return Some(format_uint_word(word));
    }
    if ty.starts_with("int") {
        return Some(format_int_word(word));
    }
    if ty == "address" {
        let padded = left_pad_word(word);
        return Some(
            format!("0x{}", &padded[padded.len().saturating_sub(40)..]).to_ascii_lowercase(),
        );
    }
    if ty == "bool" {
        let trimmed = word.trim_start_matches('0');
        return match trimmed {
            "" | "0" => Some("false".to_owned()),
            "1" => Some("true".to_owned()),
            _ => None,
        };
    }
    if ty == "bytes32" {
        return Some(normalize_hex(word));
    }
    if let Some(size) = fixed_bytes_size(ty) {
        let padded = right_pad_word(word);
        return Some(format!("0x{}", &padded[..size * 2]).to_ascii_lowercase());
    }
    None
}

/// Where the compiler leaves a function's parameters on the stack when the function is
/// entered. Solidity's two code generators disagree, and nothing in the artifacts says
/// which produced the code, so [`StepMap`] proves it from the trace instead of assuming.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ArgumentOrder {
    /// The first declared parameter is on top: the via-IR pipeline pushes them
    /// right-to-left.
    FirstOnTop,
    /// The last declared parameter is on top: the legacy pipeline pushes them
    /// left-to-right.
    LastOnTop,
}

/// What this trace proved about how a contract's compiler passes arguments.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ArgumentLayout {
    /// The parameters are the top words at the entry point, in this order.
    Ordered(ArgumentOrder),
    /// The parameters are the top words, but which is which was not established: the
    /// evidence came from a one-parameter frame, where both orders look the same.
    TopWords,
}

impl ArgumentOrder {
    /// The stack index of parameter `index` of `count`, counting from the bottom of the
    /// stack, given that the parameters occupy the top `count` words.
    #[must_use]
    fn word_index(self, index: usize, count: usize, stack_len: usize) -> usize {
        match self {
            Self::FirstOnTop => stack_len - 1 - index,
            Self::LastOnTop => stack_len - count + index,
        }
    }
}

/// What a frame's arguments are read from: the stack at its entry point, and the memory
/// a pointer among them refers into.
#[derive(Debug, Clone, Copy, Default)]
pub struct FrameState<'a> {
    pub stack: &'a [StackWord],
    /// Memory at the entry step, as one unprefixed hex string, when the backend captured
    /// it.
    pub memory: Option<&'a str>,
}

/// The arguments a function was entered with, read off the stack at its entry point.
///
/// Every parameter read here occupies exactly one word: a value type is the value, and a
/// `memory` or `storage` reference is a pointer, which memory's own layout then resolves.
/// A `calldata` slice of a dynamic type is the exception — it is a pointer *and* a length,
/// two words, and nothing in the trace proves that width the way the calldata proves the
/// order — so a function taking one reports no arguments rather than words that may be
/// misaligned.
#[must_use]
pub fn decode_arguments(
    params: &[SourceParam],
    state: FrameState<'_>,
    layout: ArgumentLayout,
    types: &SourceTypes,
    scope: Option<&str>,
) -> Vec<FrameArgument> {
    let stack = state.stack;
    if params.is_empty() || stack.len() < params.len() {
        return Vec::new();
    }
    if !params.iter().all(readable_parameter) {
        return Vec::new();
    }
    // One parameter is on top under either order; more than one needs the order proven.
    let order = match layout {
        ArgumentLayout::Ordered(order) => order,
        ArgumentLayout::TopWords if params.len() == 1 => ArgumentOrder::FirstOnTop,
        ArgumentLayout::TopWords => return Vec::new(),
    };
    params
        .iter()
        .enumerate()
        .map(|(index, param)| {
            let word = &stack[order.word_index(index, params.len(), stack.len())];
            FrameArgument {
                name: param.name.clone(),
                ty: param.ty.clone(),
                value: argument_value(param, word, state.memory, types, scope),
            }
        })
        .collect()
}

fn argument_value(
    param: &SourceParam,
    word: &str,
    memory: Option<&str>,
    types: &SourceTypes,
    scope: Option<&str>,
) -> DebugValue {
    let Ok(parsed) = parse_word(&format!("0x{}", word.trim_start_matches("0x"))) else {
        return DebugValue {
            display: "<unreadable stack word>".to_owned(),
            raw: Some(word.to_owned()),
            status: DebugValueStatus::Unavailable,
        };
    };
    let reader = ValueReader {
        memory,
        calldata: "",
        storage: None,
        layout: None,
        types,
        scope,
    };
    let raw = Some(short_hex(&parsed));
    match param.location.as_deref() {
        Some("memory") => reader.variable(&format!("{} memory", param.ty), &[word]),
        Some(location) => DebugValue {
            display: format!("<{} in {location} at {}>", param.ty, short_hex(&parsed)),
            raw,
            status: DebugValueStatus::Raw,
        },
        None => DebugValue {
            display: decode_value(value_bytes(&parsed, &param.ty), &param.ty),
            raw,
            status: DebugValueStatus::Decoded,
        },
    }
}

/// Whether a parameter is one word the debugger can place: a value, or a pointer.
pub(crate) fn readable_parameter(param: &SourceParam) -> bool {
    match param.location.as_deref() {
        // A `calldata` slice of a dynamic type is two words; see `decode_arguments`.
        Some("calldata") => !is_dynamic_type(&param.ty),
        Some(_) => true,
        None => is_value_type(&param.ty),
    }
}

/// Whether a type is stored with a length, rather than a fixed number of words.
fn is_dynamic_type(ty: &str) -> bool {
    ty == "string" || ty == "bytes" || ty.ends_with("[]")
}

/// An array type as its element type and its length: `None` for a dynamic array, which
/// carries its length in memory.
pub(crate) fn array_shape(ty: &str) -> Option<(&str, Option<usize>)> {
    let inner = ty.strip_suffix(']')?;
    let open = inner.rfind('[')?;
    let element = inner[..open].trim();
    let length = &inner[open + 1..];
    if length.is_empty() {
        return Some((element, None));
    }
    Some((element, Some(length.parse::<usize>().ok()?)))
}

/// The 32 bytes at `offset` of a memory image, when it reaches that far.
pub(crate) fn memory_word(memory: &str, offset: usize) -> Option<Word> {
    let bytes = memory_bytes(memory, offset, 32)?;
    let mut word = [0_u8; 32];
    word.copy_from_slice(&bytes);
    Some(word)
}

/// `length` bytes at `offset` of a memory image, which is two hex digits per byte.
pub(crate) fn memory_bytes(memory: &str, offset: usize, length: usize) -> Option<Vec<u8>> {
    let start = offset.checked_mul(2)?;
    let end = start.checked_add(length.checked_mul(2)?)?;
    let digits = memory.get(start..end)?;
    if !digits.bytes().all(|byte| byte.is_ascii_hexdigit()) {
        return None;
    }
    Some(
        digits
            .as_bytes()
            .chunks(2)
            .map(|pair| {
                u8::from_str_radix(std::str::from_utf8(pair).expect("ascii"), 16).expect("hex")
            })
            .collect(),
    )
}

/// A word as an offset or a length, when it fits one.
pub(crate) fn word_as_usize(word: &Word) -> Option<usize> {
    if word[..24].iter().any(|byte| *byte != 0) {
        return None;
    }
    usize::try_from(u64::from_be_bytes(word[24..].try_into().expect("8 bytes"))).ok()
}

pub(crate) fn hex_of(bytes: &[u8]) -> String {
    bytes.iter().map(|byte| format!("{byte:02x}")).collect()
}

/// The bytes of a stack word a value of type `ty` occupies: a signed integer is its own
/// width from the low end so the sign bit is the right one, fixed bytes are left-aligned,
/// and everything else reads as a whole word.
pub(crate) fn value_bytes<'a>(word: &'a Word, ty: &str) -> &'a [u8] {
    if let Some(bits) = integer_bits(ty, "int") {
        return &word[32 - bits / 8..];
    }
    if let Some(size) = fixed_bytes_size(ty) {
        return &word[..size];
    }
    word
}

/// Whether a value of this type is one stack word the debugger can decode.
#[must_use]
pub fn is_value_type(ty: &str) -> bool {
    matches!(ty, "bool" | "address" | "address payable")
        || fixed_bytes_size(ty).is_some()
        || integer_bits(ty, "uint").is_some()
        || integer_bits(ty, "int").is_some()
}

/// The width of `uint<N>`/`int<N>` in bits, the bare names counting as 256.
fn integer_bits(ty: &str, prefix: &str) -> Option<usize> {
    let rest = ty.strip_prefix(prefix)?;
    // `int8` must not match the `uint` prefix; `strip_prefix` already ruled that out.
    if rest.is_empty() {
        return Some(256);
    }
    let bits = rest.parse::<usize>().ok()?;
    (bits % 8 == 0 && (8..=256).contains(&bits)).then_some(bits)
}

fn fixed_bytes_size(ty: &str) -> Option<usize> {
    let size = ty.strip_prefix("bytes")?.parse::<usize>().ok()?;
    (1..=32).contains(&size).then_some(size)
}

fn format_uint_word(word: &str) -> String {
    let trimmed = word.trim_start_matches('0');
    if trimmed.is_empty() {
        return "0".to_owned();
    }
    if trimmed.len() <= 32 {
        return u128::from_str_radix(trimmed, 16)
            .map(|value| value.to_string())
            .unwrap_or_else(|_| format!("0x{}", trimmed.to_ascii_lowercase()));
    }
    format!("0x{}", trimmed.to_ascii_lowercase())
}

/// A two's-complement word as a signed decimal, or as hex when it does not fit `i128`.
fn format_int_word(word: &str) -> String {
    let padded = left_pad_word(word);
    let Ok(bytes) = (0..32)
        .map(|index| u8::from_str_radix(&padded[index * 2..index * 2 + 2], 16))
        .collect::<Result<Vec<_>, _>>()
    else {
        return format!("0x{}", word.to_ascii_lowercase());
    };
    if bytes[0] & 0x80 == 0 {
        return format_uint_word(word);
    }
    // Negate: invert every byte and add one, from the least significant end.
    let mut magnitude = bytes.iter().map(|byte| !byte).collect::<Vec<_>>();
    for byte in magnitude.iter_mut().rev() {
        let (sum, carry) = byte.overflowing_add(1);
        *byte = sum;
        if !carry {
            break;
        }
    }
    let hex = magnitude
        .iter()
        .map(|byte| format!("{byte:02x}"))
        .collect::<String>();
    let trimmed = hex.trim_start_matches('0');
    if trimmed.len() <= 32 {
        if let Ok(value) = u128::from_str_radix(trimmed, 16) {
            return format!("-{value}");
        }
    }
    format!("-0x{trimmed}")
}

fn storage_value(storage: &BTreeMap<String, String>, slot: u64) -> Option<String> {
    storage
        .get(&format!("0x{slot:x}"))
        .or_else(|| storage.get(&format!("{slot:x}")))
        .or_else(|| storage.get(&format!("0x{slot:064x}")))
        .or_else(|| storage.get(&format!("{slot:064x}")))
        .map(|value| normalize_hex(value))
}

fn word_from_hex_bytes(input: &str, byte_offset: usize) -> Option<String> {
    let hex = input.trim_start_matches("0x");
    let start = byte_offset.checked_mul(2)?;
    let end = start.checked_add(64)?;
    let word = hex.get(start..end)?;
    word.bytes()
        .all(|byte| byte.is_ascii_hexdigit())
        .then(|| format!("0x{}", word.to_ascii_lowercase()))
}

fn normalize_hex(value: &str) -> String {
    let value = value.trim();
    let hex = value.strip_prefix("0x").unwrap_or(value);
    format!("0x{}", hex.to_ascii_lowercase())
}

fn left_pad_word(word: &str) -> String {
    let word = word.trim_start_matches("0x").to_ascii_lowercase();
    if word.len() >= 64 {
        return word;
    }
    format!("{:0>64}", word)
}

fn right_pad_word(word: &str) -> String {
    let word = word.trim_start_matches("0x").to_ascii_lowercase();
    if word.len() >= 64 {
        return word;
    }
    format!("{:0<64}", word)
}

fn line_column_for_offset(source: &str, offset: usize) -> SourcePosition {
    let mut line = 1;
    let mut column = 1;
    for byte in source.bytes().take(offset.min(source.len())) {
        if byte == b'\n' {
            line += 1;
            column = 1;
        } else {
            column += 1;
        }
    }
    SourcePosition { line, column }
}

/// The declarations that open a body of executable code in a Solidity source.
const DECLARATION_KEYWORDS: [&str; 5] =
    ["function", "modifier", "constructor", "fallback", "receive"];

/// Finds every function-like declaration in a source file by scanning its text:
/// functions, modifiers, constructors, `fallback`, and `receive`. This is what the
/// debugger attributes steps to when the compiler emits no function boundaries.
#[must_use]
pub fn parse_source_functions(source_id: u64, source: &str) -> Vec<SourceFunction> {
    let mut functions = Vec::new();
    for keyword in DECLARATION_KEYWORDS {
        collect_declarations(source_id, source, keyword, &mut functions);
    }
    functions.sort_by_key(|function| function.declaration_start);
    functions
}

fn collect_declarations(
    source_id: u64,
    source: &str,
    keyword: &str,
    functions: &mut Vec<SourceFunction>,
) {
    let named = matches!(keyword, "function" | "modifier");
    let mut cursor = 0;
    while let Some(keyword_start) = find_solidity_keyword(source, keyword, cursor) {
        let mut index = skip_ascii_whitespace(source, keyword_start + keyword.len());
        let name = if named {
            let Some((name, name_end)) = parse_identifier(source, index) else {
                cursor = index;
                continue;
            };
            index = skip_ascii_whitespace(source, name_end);
            name
        } else {
            keyword
        };
        if source.as_bytes().get(index) != Some(&b'(') {
            cursor = index;
            continue;
        }
        let Some(params_end) = find_matching_delimiter(source, index, b'(', b')') else {
            cursor = index + 1;
            continue;
        };
        let params = parse_source_params(&source[index + 1..params_end]);
        let Some(body_start) = find_next_byte(source, params_end + 1, b'{') else {
            cursor = params_end + 1;
            continue;
        };
        // A call such as `token.receive(x);` has a `;` before the next `{`; only a
        // declaration runs straight from its parameter list into a body.
        if source[params_end + 1..body_start].contains(';') {
            cursor = params_end + 1;
            continue;
        }
        let Some(body_end) = find_matching_delimiter(source, body_start, b'{', b'}') else {
            cursor = body_start + 1;
            continue;
        };
        let (returns, modifiers) = parse_header(source, params_end + 1, body_start);
        let modifiers = if keyword == "modifier" {
            Vec::new()
        } else {
            modifiers
        };
        let returns = returns
            .into_iter()
            .map(|variable| SourceLocal {
                name: variable.name,
                ty: variable.ty,
                location: variable.location,
                declaration: variable.span,
                statement: variable.span,
                position: 0,
                scope: ByteRange {
                    start: body_start as u64,
                    end: body_end as u64,
                },
            })
            .collect();

        functions.push(SourceFunction {
            source_id,
            name: name.to_owned(),
            params,
            declaration_start: keyword_start as u64,
            declaration_line: source[..keyword_start]
                .bytes()
                .filter(|byte| *byte == b'\n')
                .count() as u64
                + 1,
            body_start: body_start as u64,
            body_end: body_end as u64,
            returns,
            locals: locals::scan_locals(source, body_start, body_end),
            has_modifiers: !modifiers.is_empty(),
            modifiers,
            placeholder: (keyword == "modifier")
                .then(|| locals::find_placeholder(source, body_start, body_end))
                .flatten(),
        });
        cursor = body_end + 1;
    }
}

/// The words a function header may carry between its parameter list and its body
/// without invoking a modifier.
const HEADER_KEYWORDS: [&str; 11] = [
    "public", "private", "internal", "external", "pure", "view", "payable", "virtual", "override",
    "returns", "constant",
];

/// The return parameters declared in a function header, and the modifiers it invokes.
///
/// The header is the text between the parameter list and the body. A `returns (...)`
/// clause holds the return parameters, each with the span the compiler reserves its slot
/// under; any other identifier that is not a visibility or mutability keyword is a
/// modifier invocation, and they run in the order written.
fn parse_header(source: &str, start: usize, end: usize) -> (Vec<locals::Declared>, Vec<String>) {
    let bytes = source.as_bytes();
    let mut returns = Vec::new();
    let mut modifiers = Vec::new();
    let mut index = start;
    while index < end {
        index = locals::skip_trivia(source, index);
        if index >= end {
            break;
        }
        let Some((word, word_end)) = parse_identifier(source, index) else {
            index += 1;
            continue;
        };
        let after = locals::skip_trivia(source, word_end);
        let arguments = (bytes.get(after) == Some(&b'('))
            .then(|| find_matching_delimiter(source, after, b'(', b')'))
            .flatten()
            .filter(|close| *close < end);
        if word == "returns" {
            if let Some(close) = arguments {
                returns = parse_return_list(source, after, close);
            }
        } else if !HEADER_KEYWORDS.contains(&word) {
            modifiers.push(word.to_owned());
        }
        index = arguments.map_or(word_end, |close| close + 1);
    }
    (returns, modifiers)
}

/// The return parameters between the parentheses at `open` and `close`.
fn parse_return_list(source: &str, open: usize, close: usize) -> Vec<locals::Declared> {
    let mut returns = Vec::new();
    let mut start = open + 1;
    let mut depth = 0_i32;
    for index in open + 1..=close {
        match source.as_bytes()[index] {
            b'(' | b'[' => depth += 1,
            b')' | b']' if depth > 0 => depth -= 1,
            b',' | b')' => {
                let component = source[start..index].trim();
                if !component.is_empty() {
                    let component_start = start
                        + (source[start..index].len() - source[start..index].trim_start().len());
                    let component_start = locals::skip_trivia(source, component_start);
                    let component_end = component_start + component.len();
                    if let Some(declared) =
                        locals::parse_return(source, component_start, component_end, returns.len())
                    {
                        returns.push(declared);
                    }
                }
                start = index + 1;
            }
            _ => {}
        }
    }
    returns
}

/// The words that may follow a parameter's type without naming it.
const PARAM_KEYWORDS: [&str; 4] = ["memory", "calldata", "storage", "payable"];

fn parse_source_params(params: &str) -> Vec<SourceParam> {
    let params = strip_param_comments(params);
    split_top_level_commas(&params)
        .into_iter()
        .enumerate()
        .filter_map(|(index, param)| {
            let param = param.trim();
            if param.is_empty() {
                return None;
            }
            let mut tokens = param.split_whitespace().collect::<Vec<_>>();
            // A parameter is `<type> [location] [name]`, and the name is optional:
            // `function f(address, uint256 amount)` leaves the first one unnamed. A
            // trailing token names the parameter only when it is an identifier, is not
            // the whole parameter, and is not one of the words that may follow a type
            // without naming it; otherwise the parameter is unnamed and takes `argN`.
            let trailing = tokens.last().copied();
            let name = match trailing {
                Some(token)
                    if tokens.len() > 1
                        && is_identifier(token)
                        && !PARAM_KEYWORDS.contains(&token) =>
                {
                    tokens.pop();
                    token.to_owned()
                }
                _ => format!("arg{index}"),
            };
            let location = tokens
                .iter()
                .find(|token| matches!(**token, "memory" | "calldata" | "storage"))
                .map(|token| (*token).to_owned());
            let ty = tokens
                .into_iter()
                .filter(|token| !PARAM_KEYWORDS.contains(token))
                .collect::<Vec<_>>()
                .join(" ");
            (!ty.is_empty()).then_some(SourceParam { name, ty, location })
        })
        .collect()
}

/// A parameter list with its comments removed.
///
/// Solidity allows a comment anywhere in a parameter list, and an unnamed parameter is
/// commonly documented with one:
///
/// ```solidity
/// function burnFrom_withCaller(
///     address caller,
///     address, //from
///     uint256, //amount
/// ) external {}
/// ```
///
/// Left in, a comment's words are split on whitespace like any other token, so `//from`
/// parses as a type and the type on the following line parses as its name. Only the text
/// between the parentheses is stripped, so no byte offset recorded for a declaration
/// moves.
fn strip_param_comments(params: &str) -> String {
    let mut output = String::with_capacity(params.len());
    let mut chars = params.chars().peekable();
    while let Some(character) = chars.next() {
        if character == '/' {
            match chars.peek() {
                Some('/') => {
                    // Drop to the end of the line, keeping the newline so the tokens on
                    // either side of it stay separate.
                    for next in chars.by_ref() {
                        if next == '\n' {
                            output.push('\n');
                            break;
                        }
                    }
                    continue;
                }
                Some('*') => {
                    chars.next();
                    let mut previous = None;
                    for next in chars.by_ref() {
                        if previous == Some('*') && next == '/' {
                            break;
                        }
                        previous = Some(next);
                    }
                    // A block comment can sit between a type and its name, so it leaves
                    // a separator behind rather than joining them.
                    output.push(' ');
                    continue;
                }
                _ => {}
            }
        }
        output.push(character);
    }
    output
}

fn split_top_level_commas(input: &str) -> Vec<&str> {
    let mut parts = Vec::new();
    let mut start = 0;
    let mut depth = 0_i32;
    for (index, byte) in input.bytes().enumerate() {
        match byte {
            b'(' | b'[' => depth += 1,
            b')' | b']' => depth -= 1,
            b',' if depth == 0 => {
                parts.push(&input[start..index]);
                start = index + 1;
            }
            _ => {}
        }
    }
    parts.push(&input[start..]);
    parts
}

pub(crate) fn find_solidity_keyword(source: &str, keyword: &str, start: usize) -> Option<usize> {
    let mut cursor = start;
    while let Some(relative) = source[cursor..].find(keyword) {
        let absolute = cursor + relative;
        let before = absolute
            .checked_sub(1)
            .and_then(|index| source.as_bytes().get(index))
            .copied();
        let after = source.as_bytes().get(absolute + keyword.len()).copied();
        if before.is_none_or(|byte| !is_identifier_byte(byte))
            && after.is_none_or(|byte| !is_identifier_byte(byte))
        {
            return Some(absolute);
        }
        cursor = absolute + keyword.len();
    }
    None
}

pub(crate) fn parse_identifier(source: &str, start: usize) -> Option<(&str, usize)> {
    let bytes = source.as_bytes();
    let first = *bytes.get(start)?;
    if !is_identifier_start_byte(first) {
        return None;
    }
    let mut end = start + 1;
    while bytes.get(end).is_some_and(|byte| is_identifier_byte(*byte)) {
        end += 1;
    }
    Some((&source[start..end], end))
}

pub(crate) fn is_identifier(input: &str) -> bool {
    let mut bytes = input.bytes();
    let Some(first) = bytes.next() else {
        return false;
    };
    is_identifier_start_byte(first) && bytes.all(is_identifier_byte)
}

fn is_identifier_start_byte(byte: u8) -> bool {
    byte == b'_' || byte.is_ascii_alphabetic()
}

pub(crate) fn is_identifier_byte(byte: u8) -> bool {
    is_identifier_start_byte(byte) || byte.is_ascii_digit()
}

pub(crate) fn skip_ascii_whitespace(source: &str, mut index: usize) -> usize {
    while source
        .as_bytes()
        .get(index)
        .is_some_and(u8::is_ascii_whitespace)
    {
        index += 1;
    }
    index
}

fn find_next_byte(source: &str, start: usize, needle: u8) -> Option<usize> {
    source
        .as_bytes()
        .iter()
        .enumerate()
        .skip(start)
        .find_map(|(index, byte)| (*byte == needle).then_some(index))
}

pub(crate) fn find_matching_delimiter(
    source: &str,
    open_index: usize,
    open: u8,
    close: u8,
) -> Option<usize> {
    let mut depth = 0_i32;
    for (index, byte) in source.bytes().enumerate().skip(open_index) {
        if byte == open {
            depth += 1;
        } else if byte == close {
            depth -= 1;
            if depth == 0 {
                return Some(index);
            }
        }
    }
    None
}

#[cfg(test)]
mod tests {
    use std::collections::BTreeMap;

    use serde_json::json;
    use soldb_core::{TraceStep, TransactionTrace};
    use soldb_ethdebug::{EthdebugInfo, Instruction, VariableLocation};

    use super::{parse_source_functions, DebugSession, DebugValueStatus};

    #[test]
    fn builds_source_steps_and_decodes_active_variables() {
        let source =
            "contract Counter {\n  function set(uint256 x) public {\n    value = x;\n  }\n}\n";
        let offset = source.find("value = x").expect("source offset") as u64;
        let trace = sample_trace();
        let mut sources = BTreeMap::new();
        sources.insert(0, "Counter.sol".to_owned());
        let mut source_contents = BTreeMap::new();
        source_contents.insert(0, source.to_owned());
        let mut variable_locations = BTreeMap::new();
        variable_locations.insert(
            0,
            vec![
                VariableLocation {
                    name: "x".to_owned(),
                    ty: "uint256".to_owned(),
                    location_type: "stack".to_owned(),
                    offset: 0,
                    pc_range: (0, 1),
                },
                VariableLocation {
                    name: "sender".to_owned(),
                    ty: "address".to_owned(),
                    location_type: "calldata".to_owned(),
                    offset: 4,
                    pc_range: (0, 1),
                },
            ],
        );
        let info = EthdebugInfo {
            compilation: json!({}),
            contract_name: "Counter".to_owned(),
            environment: "runtime".to_owned(),
            instructions: vec![Instruction {
                offset: 0,
                operation: json!({"mnemonic": "PUSH1"}),
                context: Some(json!({
                    "code": {
                        "source": {"id": 0},
                        "range": {"offset": offset, "length": 9}
                    }
                })),
            }],
            sources,
            variable_locations,
        };

        let session = DebugSession::with_ethdebug(trace, info, source_contents);
        let step = session.step(0).expect("debug step");
        assert_eq!(step.source.as_ref().expect("source").line, 3);
        assert_eq!(step.function.as_ref().expect("function").name, "set");
        assert_eq!(
            step.snapshot
                .stack
                .iter()
                .map(|word| &**word)
                .collect::<Vec<_>>(),
            ["0x2a"]
        );
        assert_eq!(step.variables[0].name, "x");
        assert_eq!(step.variables[0].value.display, "42");
        assert_eq!(step.variables[0].value.status, DebugValueStatus::Decoded);
        assert_eq!(
            step.variables[1].value.display,
            "0x1111111111111111111111111111111111111111"
        );
    }

    #[test]
    fn keeps_raw_values_when_type_is_dynamic_or_location_is_unknown() {
        let trace = sample_trace();
        let mut variable_locations = BTreeMap::new();
        variable_locations.insert(
            0,
            vec![
                VariableLocation {
                    name: "name".to_owned(),
                    ty: "string".to_owned(),
                    location_type: "stack".to_owned(),
                    offset: 0,
                    pc_range: (0, 0),
                },
                VariableLocation {
                    name: "missing".to_owned(),
                    ty: "uint256".to_owned(),
                    location_type: "stack".to_owned(),
                    offset: 99,
                    pc_range: (0, 0),
                },
            ],
        );
        let info = EthdebugInfo {
            compilation: json!({}),
            contract_name: "Counter".to_owned(),
            environment: "runtime".to_owned(),
            instructions: Vec::new(),
            sources: BTreeMap::new(),
            variable_locations,
        };

        let session = DebugSession::with_ethdebug(trace, info, BTreeMap::new());
        let step = session.step(0).expect("debug step");
        assert_eq!(step.variables[0].value.status, DebugValueStatus::Raw);
        assert_eq!(step.variables[0].value.display, "0x2a");
        assert_eq!(
            step.variables[1].value.status,
            DebugValueStatus::Unavailable
        );
    }

    #[test]
    fn attaching_ethdebug_matches_constructing_with_it_and_replaces_prior_info() {
        let source =
            "contract Counter {\n  function set(uint256 x) public {\n    value = x;\n  }\n}\n";
        let offset = source.find("value = x").expect("source offset") as u64;
        let info = EthdebugInfo {
            compilation: json!({}),
            contract_name: "Counter".to_owned(),
            environment: "runtime".to_owned(),
            instructions: vec![Instruction {
                offset: 0,
                operation: json!({"mnemonic": "PUSH1"}),
                context: Some(json!({
                    "code": {"source": {"id": 0}, "range": {"offset": offset, "length": 9}}
                })),
            }],
            sources: BTreeMap::from([(0, "Counter.sol".to_owned())]),
            variable_locations: BTreeMap::new(),
        };
        let source_contents = BTreeMap::from([(0, source.to_owned())]);

        let constructed =
            DebugSession::with_ethdebug(sample_trace(), info.clone(), source_contents.clone());
        let mut attached = DebugSession::new(sample_trace());
        assert!(attached.ethdebug.is_none());
        assert!(attached.functions.is_empty());
        attached.attach_ethdebug(info, source_contents);

        assert_eq!(attached, constructed);
        assert_eq!(attached.functions.len(), 1);
        assert_eq!(attached.functions[0].name, "set");
        assert_eq!(
            attached
                .step(0)
                .and_then(|step| step.source)
                .map(|span| span.line),
            Some(3)
        );

        // Attaching again replaces the debug info wholesale rather than merging.
        let other = EthdebugInfo {
            compilation: json!({}),
            contract_name: "Other".to_owned(),
            environment: "runtime".to_owned(),
            instructions: Vec::new(),
            sources: BTreeMap::new(),
            variable_locations: BTreeMap::new(),
        };
        attached.attach_ethdebug(other, BTreeMap::new());
        assert_eq!(
            attached
                .ethdebug
                .as_ref()
                .map(|info| info.contract_name.as_str()),
            Some("Other")
        );
        assert!(attached.functions.is_empty());
        assert!(attached.source_contents.is_empty());
        assert_eq!(attached.step(0).and_then(|step| step.source), None);
        assert_eq!(attached.trace, sample_trace());
    }

    #[test]
    fn parses_source_functions_with_tuple_and_array_params() {
        let functions = parse_source_functions(
            7,
            "contract C { function submit(Person memory p, uint256[2] memory xs) public {} }",
        );
        assert_eq!(functions[0].source_id, 7);
        assert_eq!(functions[0].name, "submit");
        assert_eq!(functions[0].params[0].name, "p");
        assert_eq!(functions[0].params[0].ty, "Person");
        assert_eq!(functions[0].params[1].name, "xs");
        assert_eq!(functions[0].params[1].ty, "uint256[2]");
    }

    #[test]
    fn unnamed_params_documented_by_comments_do_not_become_parameters() {
        // The shape this comes from, verbatim: unnamed parameters whose names live in
        // trailing comments. Before comments were stripped, `//from` parsed as a type and
        // the type on the next line parsed as its name.
        let functions = parse_source_functions(
            0,
            "contract C {\n    function burnFrom_withCaller(\n        address caller,\n                     address, //from\n        uint256, //amount\n        bytes32 //h\n    ) external              view returns (bool) {}\n}",
        );
        let params = &functions[0].params;
        assert_eq!(functions[0].name, "burnFrom_withCaller");
        assert_eq!(params.len(), 4);
        assert_eq!(
            (params[0].ty.as_str(), params[0].name.as_str()),
            ("address", "caller")
        );
        assert_eq!(
            (params[1].ty.as_str(), params[1].name.as_str()),
            ("address", "arg1")
        );
        assert_eq!(
            (params[2].ty.as_str(), params[2].name.as_str()),
            ("uint256", "arg2")
        );
        assert_eq!(
            (params[3].ty.as_str(), params[3].name.as_str()),
            ("bytes32", "arg3")
        );
    }

    #[test]
    fn unnamed_params_take_positional_names_not_their_type() {
        let functions = parse_source_functions(
            0,
            "contract C { function burn(address from, uint256 amount, bytes32, bytes memory              signature) public {} }",
        );
        let params = &functions[0].params;
        assert_eq!(
            (params[1].ty.as_str(), params[1].name.as_str()),
            ("uint256", "amount")
        );
        // Previously reported as `bytes32 bytes32`, the type standing in as the name.
        assert_eq!(
            (params[2].ty.as_str(), params[2].name.as_str()),
            ("bytes32", "arg2")
        );
        assert_eq!(
            (params[3].ty.as_str(), params[3].name.as_str()),
            ("bytes", "signature")
        );
        assert_eq!(params[3].location.as_deref(), Some("memory"));
    }

    #[test]
    fn a_trailing_declaration_keyword_does_not_name_a_param() {
        let functions = parse_source_functions(
            0,
            "contract C { function f(string memory, address payable, bytes calldata data) public              {} }",
        );
        let params = &functions[0].params;
        assert_eq!(
            (params[0].ty.as_str(), params[0].name.as_str()),
            ("string", "arg0")
        );
        assert_eq!(params[0].location.as_deref(), Some("memory"));
        assert_eq!(
            (params[1].ty.as_str(), params[1].name.as_str()),
            ("address", "arg1")
        );
        assert_eq!(
            (params[2].ty.as_str(), params[2].name.as_str()),
            ("bytes", "data")
        );
        assert_eq!(params[2].location.as_deref(), Some("calldata"));
    }

    #[test]
    fn block_comments_in_a_param_list_separate_rather_than_join() {
        let functions = parse_source_functions(
            0,
            "contract C { function f(uint256 /* wei */ amount, address /*to*/) public {} }",
        );
        let params = &functions[0].params;
        assert_eq!(
            (params[0].ty.as_str(), params[0].name.as_str()),
            ("uint256", "amount")
        );
        assert_eq!(
            (params[1].ty.as_str(), params[1].name.as_str()),
            ("address", "arg1")
        );
    }

    #[test]
    fn stripping_comments_leaves_declaration_offsets_alone() {
        // Only the text between the parentheses is stripped, so the offsets the PC
        // mapping reads still point into the original source.
        let source = "contract C {\n    function f(address, //from\n        uint256 amount\n                          ) public {}\n}";
        let functions = parse_source_functions(0, source);
        let start = functions[0].declaration_start as usize;
        assert!(source[start..].starts_with("function f("));
        assert_eq!(functions[0].declaration_line, 2);
        assert_eq!(source.as_bytes()[functions[0].body_end as usize], b'}');
    }

    fn sample_trace() -> TransactionTrace {
        TransactionTrace {
            tx_hash: Some("0xabc".to_owned()),
            from_addr: "0x1".to_owned(),
            to_addr: Some("0x2".to_owned()),
            value: "0".to_owned(),
            input_data: format!(
                "0x12345678{}",
                "0000000000000000000000001111111111111111111111111111111111111111"
            ),
            gas_used: 1,
            output: "0x".to_owned(),
            success: true,
            error: None,
            debug_trace_available: true,
            contract_address: None,
            backend: Some("debug-rpc".to_owned()),
            capabilities: Default::default(),
            artifacts: Default::default(),
            steps: vec![TraceStep {
                pc: 0,
                op: "PUSH1".into(),
                gas: 1,
                gas_cost: 0,
                depth: 0,
                stack: vec!["0x2a".into()],
                memory: Some(format!("{}{}", "00".repeat(32), "2a".repeat(32))),
                storage: Some(BTreeMap::from([("0x0".to_owned(), "0x2a".to_owned())])),
                error: None,
                snapshot: Default::default(),
            }],
        }
    }
    /// Solidity memory: a `string` is a length then its bytes; an array is a length then
    /// one word per element; a fixed-size array is those words with no length.
    fn memory_image(words: &[&str]) -> String {
        words
            .iter()
            .map(|word| format!("{:0>64}", word.trim_start_matches("0x")))
            .collect()
    }

    #[test]
    fn reads_arguments_that_live_in_memory() {
        // 0x00: "hello" as a string (length 5), 0x40: [7, 8, 9] as a dynamic array.
        let memory = memory_image(&[
            "5",
            "68656c6c6f000000000000000000000000000000000000000000000000000000",
            "3",
            "7",
            "8",
            "9",
        ]);
        let types = super::SourceTypes::default();
        let reader = super::ValueReader {
            memory: Some(&memory),
            calldata: "",
            storage: None,
            layout: None,
            types: &types,
            scope: None,
        };
        let read = |offset: usize, ty: &str| reader.read_memory(offset, ty, 0);
        assert_eq!(read(0, "string").as_deref(), Some("\"hello\""));
        assert_eq!(read(0, "bytes").as_deref(), Some("0x68656c6c6f"));
        assert_eq!(read(64, "uint256[]").as_deref(), Some("[7, 8, 9]"));
        // A fixed-size array has no length word: it starts at the pointer.
        assert_eq!(read(96, "uint256[2]").as_deref(), Some("[7, 8]"));
        // Beyond what the step captured: no guess, and the caller says where the pointer
        // pointed instead. An array of strings follows each element's pointer, and says
        // where one pointed when it cannot.
        assert_eq!(read(4096, "string"), None);
        assert_eq!(
            read(64, "string[]").as_deref(),
            Some("[<string at 0x7>, <string at 0x8>, <string at 0x9>]")
        );
    }

    #[test]
    fn a_parameter_is_readable_only_when_its_width_is_known() {
        let param = |ty: &str, location: Option<&str>| super::SourceParam {
            name: "x".to_owned(),
            ty: ty.to_owned(),
            location: location.map(str::to_owned),
        };
        assert!(super::readable_parameter(&param("uint256", None)));
        assert!(super::readable_parameter(&param("string", Some("memory"))));
        assert!(super::readable_parameter(&param("Config", Some("storage"))));
        // One word: a calldata slice of a fixed-size type is a pointer.
        assert!(super::readable_parameter(&param(
            "uint256[2]",
            Some("calldata")
        )));
        // Two words, and nothing proves that width: not read.
        assert!(!super::readable_parameter(&param(
            "bytes",
            Some("calldata")
        )));
        assert!(!super::readable_parameter(&param(
            "uint256[]",
            Some("calldata")
        )));
        // A struct by value is not one word of anything the debugger can decode.
        assert!(!super::readable_parameter(&param("Config", None)));
    }
}