behavior-contracts 0.1.2

//! expr — reference evaluator for expression-ir.md (normative).
//!
//! Value model: int = checked i64 (overflow → Failure, never wrap/panic),
//! float = f64 (NaN/±Inf → Failure), string / bool / null / arr / obj.
//!
//! Normative points (§6/§8 traps):
//!   - int arithmetic is checked i64 (overflow = INT_OVERFLOW).
//!   - a result that becomes NaN/±Inf is a Failure.
//!   - mod is truncated division (sign follows dividend; Rust `%` already does this).
//!   - string comparison is code-point order (Rust `str` Ord = UTF-8 byte order = code point).
//!   - and/or/coalesce/cond short-circuit.
//!   - unknown operators fail closed.

use crate::value::{deep_equals, Value};
use serde_json::Value as J;

/// Expression failure codes (PROTOCOL §3.1).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ExprFailureCode {
    IntOverflow,
    NanOrInf,
    ModZero,
    PrecisionLoss,
    TypeMismatch,
    NullRef,
    MissingProp,
    UnknownBinding,
    UnknownOp,
    InvalidNode,
    InvalidLiteral,
}

impl ExprFailureCode {
    /// The stable string form used for conformance code matching.
    pub fn as_str(self) -> &'static str {
        match self {
            ExprFailureCode::IntOverflow => "INT_OVERFLOW",
            ExprFailureCode::NanOrInf => "NAN_OR_INF",
            ExprFailureCode::ModZero => "MOD_ZERO",
            ExprFailureCode::PrecisionLoss => "PRECISION_LOSS",
            ExprFailureCode::TypeMismatch => "TYPE_MISMATCH",
            ExprFailureCode::NullRef => "NULL_REF",
            ExprFailureCode::MissingProp => "MISSING_PROP",
            ExprFailureCode::UnknownBinding => "UNKNOWN_BINDING",
            ExprFailureCode::UnknownOp => "UNKNOWN_OP",
            ExprFailureCode::InvalidNode => "INVALID_NODE",
            ExprFailureCode::InvalidLiteral => "INVALID_LITERAL",
        }
    }
}

#[derive(Debug, Clone)]
pub struct ExprFailure {
    pub code: ExprFailureCode,
    pub message: String,
}

impl std::fmt::Display for ExprFailure {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}: {}", self.code.as_str(), self.message)
    }
}
impl std::error::Error for ExprFailure {}

type R = Result<Value, ExprFailure>;

fn fail<T>(code: ExprFailureCode, message: impl Into<String>) -> Result<T, ExprFailure> {
    Err(ExprFailure {
        code,
        message: message.into(),
    })
}

const WIDEN_EXACT: i64 = 1 << 53; // ±2^53 exact in f64

fn check_finite(v: f64) -> R {
    if v.is_finite() {
        Ok(Value::Float(v))
    } else {
        fail(ExprFailureCode::NanOrInf, format!("non-finite float: {v}"))
    }
}

fn widen_to_float(v: &Value) -> Result<f64, ExprFailure> {
    match v {
        Value::Float(f) => Ok(*f),
        Value::Int(i) => {
            if *i > WIDEN_EXACT || *i < -WIDEN_EXACT {
                fail(
                    ExprFailureCode::PrecisionLoss,
                    format!("int {i} exceeds exact float range (±2^53)"),
                )
            } else {
                Ok(*i as f64)
            }
        }
        other => fail(
            ExprFailureCode::TypeMismatch,
            format!("numeric operand expected, got {}", other.type_name()),
        ),
    }
}

/// Compare two strings by code point. Rust `str` Ord IS UTF-8 byte order which
/// equals Unicode code-point order, so we can delegate to the native comparison.
/// Kept as a named function to make the normative choice explicit.
pub fn cmp_code_points(a: &str, b: &str) -> std::cmp::Ordering {
    a.cmp(b)
}

fn require_bool(v: &Value, ctx: &str) -> Result<bool, ExprFailure> {
    match v {
        Value::Bool(b) => Ok(*b),
        other => fail(
            ExprFailureCode::TypeMismatch,
            format!(
                "{ctx}: bool expected, got {} (no truthiness)",
                other.type_name()
            ),
        ),
    }
}

/// Evaluate an IR expression node against a scope, producing a runtime [`Value`].
pub fn evaluate(node: &J, scope: &[(String, Value)]) -> R {
    match node {
        J::Null => Ok(Value::Null),
        J::Bool(b) => Ok(Value::Bool(*b)),
        J::String(s) => Ok(Value::Str(s.clone())),
        J::Number(n) => {
            // Classification (§2.3): integral → int (must be safe-range), fractional → float.
            if n.is_i64() {
                let i = n.as_i64().unwrap();
                // safe integer check: |i| <= 2^53-1
                const SAFE: i64 = 9_007_199_254_740_991;
                if !(-SAFE..=SAFE).contains(&i) {
                    return fail(
                        ExprFailureCode::InvalidLiteral,
                        format!("integral literal {i} exceeds safe range; use {{int:\"…\"}}"),
                    );
                }
                Ok(Value::Int(i))
            } else if n.is_u64() {
                // u64 beyond i64 => beyond safe range
                fail(
                    ExprFailureCode::InvalidLiteral,
                    format!("integral literal {n} exceeds safe range; use {{int:\"…\"}}"),
                )
            } else {
                let f = n.as_f64().ok_or(ExprFailure {
                    code: ExprFailureCode::InvalidLiteral,
                    message: format!("bad number literal {n}"),
                })?;
                check_finite(f)
            }
        }
        J::Array(_) => fail(
            ExprFailureCode::InvalidNode,
            "bare array is not an expression (use {arr:[...]})",
        ),
        J::Object(map) => {
            if map.len() != 1 {
                let keys: Vec<&str> = map.keys().map(|s| s.as_str()).collect();
                return fail(
                    ExprFailureCode::InvalidNode,
                    format!(
                        "operator node must have exactly one key, got [{}]",
                        keys.join(", ")
                    ),
                );
            }
            let (op, arg) = map.iter().next().unwrap();
            eval_op(op, arg, scope)
        }
    }
}

fn eval_op(op: &str, arg: &J, scope: &[(String, Value)]) -> R {
    match op {
        "int" => {
            let s = arg.as_str().ok_or_else(|| ExprFailure {
                code: ExprFailureCode::InvalidNode,
                message: "{int:…} expects a string".into(),
            })?;
            match s.parse::<i64>() {
                Ok(v) => Ok(Value::Int(v)),
                Err(_) => {
                    // distinguish overflow (valid decimal beyond i64) from garbage.
                    if s.trim_start_matches('-')
                        .chars()
                        .all(|c| c.is_ascii_digit())
                        && !s.is_empty()
                        && s != "-"
                    {
                        fail(ExprFailureCode::IntOverflow, format!("i64 overflow: {s}"))
                    } else {
                        fail(
                            ExprFailureCode::InvalidLiteral,
                            format!("invalid int literal: {s}"),
                        )
                    }
                }
            }
        }
        "float" => {
            let n = arg.as_f64().ok_or_else(|| ExprFailure {
                code: ExprFailureCode::InvalidNode,
                message: "{float:…} expects a number".into(),
            })?;
            check_finite(n)
        }
        "ref" | "refOpt" => eval_ref(op, arg, scope),
        "obj" => {
            let m = arg.as_object().ok_or_else(|| ExprFailure {
                code: ExprFailureCode::InvalidNode,
                message: "{obj:…} expects an object".into(),
            })?;
            let mut out = Vec::with_capacity(m.len());
            for (k, v) in m {
                out.push((k.clone(), evaluate(v, scope)?));
            }
            Ok(Value::Obj(out))
        }
        "arr" => {
            let a = arg_array(op, arg)?;
            let mut out = Vec::with_capacity(a.len());
            for e in a {
                out.push(evaluate(e, scope)?);
            }
            Ok(Value::Arr(out))
        }
        "add" | "sub" | "mul" => {
            let (a, b) = eval_binary(op, arg, scope)?;
            match (&a, &b) {
                (Value::Int(x), Value::Int(y)) => {
                    let r = match op {
                        "add" => x.checked_add(*y),
                        "sub" => x.checked_sub(*y),
                        _ => x.checked_mul(*y),
                    };
                    match r {
                        Some(v) => Ok(Value::Int(v)),
                        None => fail(
                            ExprFailureCode::IntOverflow,
                            format!("i64 overflow in {op}"),
                        ),
                    }
                }
                (Value::Float(x), Value::Float(y)) => {
                    let r = match op {
                        "add" => x + y,
                        "sub" => x - y,
                        _ => x * y,
                    };
                    check_finite(r)
                }
                _ => fail(
                    ExprFailureCode::TypeMismatch,
                    format!(
                        "{op}: int×int or float×float (got {}×{})",
                        a.type_name(),
                        b.type_name()
                    ),
                ),
            }
        }
        "neg" => {
            let a = evaluate(arg_unary(op, arg)?, scope)?;
            match a {
                Value::Int(i) => match i.checked_neg() {
                    Some(v) => Ok(Value::Int(v)),
                    None => fail(ExprFailureCode::IntOverflow, "i64 overflow in neg"),
                },
                Value::Float(f) => check_finite(-f),
                other => fail(
                    ExprFailureCode::TypeMismatch,
                    format!("neg: numeric expected, got {}", other.type_name()),
                ),
            }
        }
        "div" => {
            let (a, b) = eval_binary(op, arg, scope)?;
            let fa = widen_to_float(&a)?;
            let fb = widen_to_float(&b)?;
            check_finite(fa / fb)
        }
        "mod" => {
            let (a, b) = eval_binary(op, arg, scope)?;
            match (&a, &b) {
                (Value::Int(x), Value::Int(y)) => {
                    if *y == 0 {
                        return fail(ExprFailureCode::ModZero, "int mod by zero");
                    }
                    // Rust `%` is truncated (sign follows dividend), matching spec.
                    // checked_rem guards the i64::MIN % -1 overflow edge.
                    match x.checked_rem(*y) {
                        Some(v) => Ok(Value::Int(v)),
                        None => fail(ExprFailureCode::IntOverflow, "i64 overflow in mod"),
                    }
                }
                (Value::Float(x), Value::Float(y)) => check_finite(x % y),
                _ => fail(
                    ExprFailureCode::TypeMismatch,
                    format!(
                        "mod: int×int or float×float (got {}×{})",
                        a.type_name(),
                        b.type_name()
                    ),
                ),
            }
        }
        "concat" => {
            let a = arg_array(op, arg)?;
            let mut s = String::new();
            for e in a {
                match evaluate(e, scope)? {
                    Value::Str(p) => s.push_str(&p),
                    other => {
                        return fail(
                            ExprFailureCode::TypeMismatch,
                            format!(
                                "concat: strings only (got {}; no implicit toString)",
                                other.type_name()
                            ),
                        )
                    }
                }
            }
            Ok(Value::Str(s))
        }
        "eq" | "ne" => {
            let (a, b) = eval_binary(op, arg, scope)?;
            let equal = value_equals(&a, &b)?;
            Ok(Value::Bool(if op == "eq" { equal } else { !equal }))
        }
        "lt" | "le" | "gt" | "ge" => {
            use std::cmp::Ordering;
            let (a, b) = eval_binary(op, arg, scope)?;
            let c: Ordering = match (&a, &b) {
                (Value::Int(x), Value::Int(y)) => x.cmp(y),
                (Value::Float(x), Value::Float(y)) => x.partial_cmp(y).unwrap_or(Ordering::Equal),
                (Value::Str(x), Value::Str(y)) => cmp_code_points(x, y),
                _ => {
                    return fail(
                        ExprFailureCode::TypeMismatch,
                        format!(
                            "{op}: same-typed int/float/string only (got {}×{})",
                            a.type_name(),
                            b.type_name()
                        ),
                    )
                }
            };
            let res = match op {
                "lt" => c == Ordering::Less,
                "le" => c != Ordering::Greater,
                "gt" => c == Ordering::Greater,
                _ => c != Ordering::Less,
            };
            Ok(Value::Bool(res))
        }
        "and" | "or" => {
            let (ea, eb) = raw_binary(op, arg)?;
            let a = require_bool(&evaluate(ea, scope)?, op)?;
            if op == "and" && !a {
                return Ok(Value::Bool(false));
            }
            if op == "or" && a {
                return Ok(Value::Bool(true));
            }
            Ok(Value::Bool(require_bool(&evaluate(eb, scope)?, op)?))
        }
        "not" => {
            let a = require_bool(&evaluate(arg_unary(op, arg)?, scope)?, "not")?;
            Ok(Value::Bool(!a))
        }
        "coalesce" => {
            let (ea, eb) = raw_binary(op, arg)?;
            let a = evaluate(ea, scope)?;
            match a {
                Value::Null => evaluate(eb, scope),
                other => Ok(other),
            }
        }
        "cond" => {
            let a = arg
                .as_array()
                .filter(|a| a.len() == 3)
                .ok_or_else(|| ExprFailure {
                    code: ExprFailureCode::InvalidNode,
                    message: "cond expects [c, t, e]".into(),
                })?;
            let c = require_bool(&evaluate(&a[0], scope)?, "cond")?;
            evaluate(if c { &a[1] } else { &a[2] }, scope)
        }
        "len" => {
            let a = evaluate(arg_unary(op, arg)?, scope)?;
            match a {
                Value::Arr(v) => Ok(Value::Int(v.len() as i64)),
                other => fail(
                    ExprFailureCode::TypeMismatch,
                    format!(
                        "len: arrays only (string length is not v1; got {})",
                        other.type_name()
                    ),
                ),
            }
        }
        _ => fail(
            ExprFailureCode::UnknownOp,
            format!("unknown operator: {op} (fail-closed)"),
        ),
    }
}

fn eval_ref(op: &str, arg: &J, scope: &[(String, Value)]) -> R {
    let path = arg_array(op, arg)?;
    if path.is_empty() || !path.iter().all(|p| p.is_string()) {
        return fail(
            ExprFailureCode::InvalidNode,
            format!("{op} expects a non-empty string path"),
        );
    }
    let head = path[0].as_str().unwrap();
    let mut cur: Value = match scope.iter().find(|(k, _)| k == head) {
        Some((_, v)) => v.clone(),
        None => {
            return fail(
                ExprFailureCode::UnknownBinding,
                format!("unknown binding: {head}"),
            )
        }
    };
    for seg_node in &path[1..] {
        let seg = seg_node.as_str().unwrap();
        match cur {
            Value::Null => {
                if op == "refOpt" {
                    return Ok(Value::Null);
                }
                return fail(
                    ExprFailureCode::NullRef,
                    format!("null intermediate at .{seg} (use ?.)"),
                );
            }
            Value::Obj(ref pairs) => match pairs.iter().find(|(k, _)| k == seg) {
                Some((_, v)) => {
                    let next = v.clone();
                    cur = next;
                }
                None => {
                    return fail(
                        ExprFailureCode::MissingProp,
                        format!("missing property .{seg}"),
                    )
                }
            },
            ref other => {
                return fail(
                    ExprFailureCode::TypeMismatch,
                    format!("cannot access .{seg} on {}", other.type_name()),
                )
            }
        }
    }
    Ok(cur)
}

/// Equality for eq/ne: null-vs-null allowed; otherwise same scalar type only.
fn value_equals(a: &Value, b: &Value) -> Result<bool, ExprFailure> {
    if matches!(a, Value::Null) || matches!(b, Value::Null) {
        return Ok(matches!(a, Value::Null) && matches!(b, Value::Null));
    }
    let ta = a.type_name();
    let tb = b.type_name();
    if ta != tb {
        return fail(
            ExprFailureCode::TypeMismatch,
            format!("eq/ne: same type only (got {ta}×{tb})"),
        );
    }
    if ta == "arr" || ta == "obj" {
        return fail(
            ExprFailureCode::TypeMismatch,
            "eq/ne: obj/arr equality is undefined in v1",
        );
    }
    Ok(deep_equals(a, b))
}

// ── arg helpers ──────────────────────────────────────────────────────────────
fn arg_array<'a>(op: &str, arg: &'a J) -> Result<&'a Vec<J>, ExprFailure> {
    arg.as_array().ok_or_else(|| ExprFailure {
        code: ExprFailureCode::InvalidNode,
        message: format!("{op} expects an args array"),
    })
}
fn arg_unary<'a>(op: &str, arg: &'a J) -> Result<&'a J, ExprFailure> {
    let a = arg_array(op, arg)?;
    if a.len() != 1 {
        return Err(ExprFailure {
            code: ExprFailureCode::InvalidNode,
            message: format!("{op} expects 1 arg"),
        });
    }
    Ok(&a[0])
}
fn raw_binary<'a>(op: &str, arg: &'a J) -> Result<(&'a J, &'a J), ExprFailure> {
    let a = arg_array(op, arg)?;
    if a.len() != 2 {
        return Err(ExprFailure {
            code: ExprFailureCode::InvalidNode,
            message: format!("{op} expects 2 args"),
        });
    }
    Ok((&a[0], &a[1]))
}
fn eval_binary(
    op: &str,
    arg: &J,
    scope: &[(String, Value)],
) -> Result<(Value, Value), ExprFailure> {
    let (ea, eb) = raw_binary(op, arg)?;
    Ok((evaluate(ea, scope)?, evaluate(eb, scope)?))
}