tessellate 0.1.0

use anstyle::{AnsiColor, Style};
use std::borrow::Cow;
use std::collections::{BTreeMap, BTreeSet};
use std::fmt::{Arguments, Write as _};
use std::path::Path;
use tess::ast::{Cardinality, Effect};
use tess::compiler::normalize_name;
use tess::engine::{
    Candidate, DecisionStatus, Evaluation, Input, RuleOutcome, RuleTrace, SuppressionReason,
};
use tess::value::Value;
use tess::{CompiledProgram, SourceFile, Span};

const CASE_STYLE: Style = AnsiColor::Cyan.on_default().bold();
const NAME_STYLE: Style = AnsiColor::Blue.on_default().bold();
const SUCCESS_STYLE: Style = AnsiColor::Green.on_default().bold();
const INCOMPLETE_STYLE: Style = AnsiColor::Yellow.on_default().bold();
const CONFLICT_STYLE: Style = AnsiColor::Red.on_default().bold();
const SECTION_STYLE: Style = AnsiColor::Cyan.on_default().bold();
const LABEL_STYLE: Style = AnsiColor::BrightBlack.on_default();
const LOCATION_STYLE: Style = AnsiColor::BrightBlack.on_default();
const MARKER_STYLE: Style = AnsiColor::Cyan.on_default().bold();
const METADATA_VALUE_COLUMN: usize = 15;

#[derive(Clone, Copy)]
struct Styles {
    color: bool,
}

impl Styles {
    const fn new(color: bool) -> Self {
        Self { color }
    }

    fn write(self, output: &mut String, style: Style, value: Arguments<'_>) {
        if self.color {
            let _ = write!(output, "{style}{value}{style:#}");
        } else {
            let _ = output.write_fmt(value);
        }
    }
}

/// Render one evaluation as a source-oriented explanation for humans.
///
/// The serialized evaluation deliberately stays location-free. Human-facing
/// tools can recover declaration spans from the compiled program without
/// changing the stable JSON result schema.
pub(crate) fn render_evaluation(
    program: &CompiledProgram,
    input: &Input,
    evaluation: &Evaluation,
    case_name: Option<&str>,
    color: bool,
) -> String {
    render_evaluations(
        program,
        input,
        std::slice::from_ref(evaluation),
        case_name,
        color,
    )
}

/// Render all evaluations for one input in a single case-shaped block.
///
/// Context is intentionally emitted once so a case with several expectations
/// reads as one input followed by a nested list of decision results.
pub(crate) fn render_evaluations(
    program: &CompiledProgram,
    input: &Input,
    evaluations: &[Evaluation],
    case_name: Option<&str>,
    color: bool,
) -> String {
    let styles = Styles::new(color);
    let mut output = String::new();
    write_context(&mut output, input, case_name, styles);

    for evaluation in evaluations {
        output.push('\n');
        write_evaluation(&mut output, program, evaluation, styles);
    }

    while output.ends_with("\n\n") {
        output.pop();
    }
    if !output.ends_with('\n') {
        output.push('\n');
    }
    output
}

fn write_context(output: &mut String, input: &Input, case_name: Option<&str>, styles: Styles) {
    let heading = case_name.unwrap_or("input");
    styles.write(output, CASE_STYLE, format_args!("{heading}"));
    output.push('\n');

    if input.bindings.is_empty() {
        output.push_str("  (no entity bindings)\n");
        return;
    }

    for (binding_name, binding) in &input.bindings {
        write_indent(output, 2);
        output.push_str("let ");
        styles.write(output, NAME_STYLE, format_args!("{binding_name}"));
        output.push_str(" = ");
        styles.write(output, NAME_STYLE, format_args!("{}", binding.entity));
        if binding.fields.is_empty() {
            output.push_str(" {}\n");
            continue;
        }
        output.push_str(" {\n");
        for (field, value) in &binding.fields {
            let _ = writeln!(output, "    {field}: {value},");
        }
        output.push_str("  }\n");
    }
}

fn write_evaluation(
    output: &mut String,
    program: &CompiledProgram,
    evaluation: &Evaluation,
    styles: Styles,
) {
    write_status(output, evaluation, styles);
    write_contract(output, evaluation, styles);

    let undefined_decision = match evaluation.result.status {
        DecisionStatus::Undefined => program.decision(&evaluation.result.decision),
        _ => None,
    };
    if let Some(decision) = undefined_decision {
        output.push('\n');
        write_snippet(
            output,
            program.source(),
            decision.span,
            "this decision requires exactly one value, but no candidate survived",
            4,
            styles,
        );
    }

    let mut explained_rules = BTreeSet::new();
    write_support(output, program, evaluation, &mut explained_rules, styles);
    write_suppressed(output, program, evaluation, &mut explained_rules, styles);
    write_blocked_rules(output, program, evaluation, &mut explained_rules, styles);
    write_other_applied_rules(output, program, evaluation, &explained_rules, styles);
    write_skipped_rules(output, program, evaluation, styles);
    write_help(output, evaluation, styles);
}

fn write_status(output: &mut String, evaluation: &Evaluation, styles: Styles) {
    let (style, status) = match &evaluation.result.status {
        DecisionStatus::Resolved { values } => (
            SUCCESS_STYLE,
            format!("✓ {} = {}", evaluation.query, resolved_values(values)),
        ),
        DecisionStatus::Undefined => (
            INCOMPLETE_STYLE,
            format!("○ {} = undefined", evaluation.query),
        ),
        DecisionStatus::Unknown { .. } => (
            INCOMPLETE_STYLE,
            format!("? {} = unknown", evaluation.query),
        ),
        DecisionStatus::Conflict { values, .. } => (
            CONFLICT_STYLE,
            format!(
                "✗ {} = conflict [{}]",
                evaluation.query,
                display_values(values)
            ),
        ),
    };
    write_indent(output, 2);
    styles.write(output, style, format_args!("{status}"));
    output.push('\n');
}

fn write_contract(output: &mut String, evaluation: &Evaluation, styles: Styles) {
    let distinct = evaluation
        .result
        .candidates
        .iter()
        .map(|candidate| &candidate.value)
        .collect::<BTreeSet<_>>()
        .len();
    let support = evaluation.result.candidates.len();
    write_metadata(
        output,
        4,
        "cardinality",
        format_args!("{}", cardinality_contract(evaluation.result.cardinality)),
        styles,
    );
    write_metadata(
        output,
        4,
        "candidates",
        format_args!(
            "{} · {} · {} overridden",
            counted(support, "rule candidate", "rule candidates"),
            counted(distinct, "distinct value", "distinct values"),
            evaluation.result.suppressed.len()
        ),
        styles,
    );

    match &evaluation.result.status {
        DecisionStatus::Resolved { values } if values.is_empty() => write_metadata(
            output,
            4,
            "reason",
            format_args!("no value remained, which this cardinality permits"),
            styles,
        ),
        DecisionStatus::Resolved { .. } if support > distinct => write_metadata(
            output,
            4,
            "reason",
            format_args!(
                "{} agreed on {}; equal values were merged",
                counted(support, "rule", "rules"),
                counted(distinct, "distinct value", "distinct values")
            ),
            styles,
        ),
        DecisionStatus::Resolved { .. } => write_metadata(
            output,
            4,
            "reason",
            format_args!(
                "{} remained after explicit overrides",
                counted(distinct, "distinct value", "distinct values")
            ),
            styles,
        ),
        DecisionStatus::Undefined => write_metadata(
            output,
            4,
            "reason",
            format_args!("no candidate survived for a decision that requires one"),
            styles,
        ),
        DecisionStatus::Unknown { missing } => {
            write_metadata(
                output,
                4,
                "missing",
                format_args!("{}", missing.join(", ")),
                styles,
            );
            write_metadata(
                output,
                4,
                "reason",
                format_args!("missing input can still change which rules or overrides apply"),
                styles,
            );
        }
        DecisionStatus::Conflict { reasons, .. } => {
            for reason in reasons {
                write_metadata(output, 4, "reason", format_args!("{reason}"), styles);
            }
            write_metadata(
                output,
                4,
                "note",
                format_args!("source order never chooses between different values"),
                styles,
            );
        }
    }
}

fn write_support(
    output: &mut String,
    program: &CompiledProgram,
    evaluation: &Evaluation,
    explained_rules: &mut BTreeSet<String>,
    styles: Styles,
) {
    if evaluation.result.candidates.is_empty() {
        return;
    }
    let mut groups = BTreeMap::<&Value, Vec<&Candidate>>::new();
    for candidate in &evaluation.result.candidates {
        groups.entry(&candidate.value).or_default().push(candidate);
    }

    for (value, candidates) in groups {
        output.push('\n');
        let merged = candidates.len() > 1;
        let qualifier = match evaluation.result.status {
            DecisionStatus::Unknown { .. } => "provisional candidate",
            DecisionStatus::Conflict { .. } => "conflicting candidate",
            DecisionStatus::Resolved { .. } if merged => "merged candidate",
            DecisionStatus::Resolved { .. } | DecisionStatus::Undefined => "selected candidate",
        };
        write_section(output, 4, format_args!("{qualifier} `{value}`"), styles);
        for candidate in &candidates {
            explained_rules.insert(normalize_name(&candidate.rule));
            let (span, exact_effect) =
                candidate_location(program, &candidate.rule, &evaluation.result.decision);
            let label = if exact_effect {
                format!("proposed `{value}` by rule `{}`", candidate.rule)
            } else {
                format!("rule `{}` supports `{value}`", candidate.rule)
            };
            if let Some(span) = span {
                write_snippet(output, program.source(), span, &label, 6, styles);
            } else {
                write_metadata(
                    output,
                    6,
                    "rule",
                    format_args!("{} proposed `{value}`", candidate.rule),
                    styles,
                );
            }
            write_when_note(output, evaluation, &candidate.rule, styles);
            if let Some(source) = &candidate.source {
                write_metadata(output, 6, "source", format_args!("{source}"), styles);
            }
        }
        if merged {
            write_metadata(
                output,
                6,
                "note",
                format_args!("equal values are merged and count once toward cardinality"),
                styles,
            );
        }
    }
}

fn write_suppressed(
    output: &mut String,
    program: &CompiledProgram,
    evaluation: &Evaluation,
    explained_rules: &mut BTreeSet<String>,
    styles: Styles,
) {
    for suppressed in &evaluation.result.suppressed {
        output.push('\n');
        let candidate = &suppressed.candidate;
        explained_rules.insert(normalize_name(&candidate.rule));
        let SuppressionReason::ExplicitOverride { by } = &suppressed.reason;
        explained_rules.insert(normalize_name(by));
        write_section(
            output,
            4,
            format_args!("overridden candidate `{}`", candidate.value),
            styles,
        );

        let (candidate_span, exact_effect) =
            candidate_location(program, &candidate.rule, &evaluation.result.decision);
        let candidate_label = if exact_effect {
            format!("proposed by rule `{}`", candidate.rule)
        } else {
            format!("candidate came from rule `{}`", candidate.rule)
        };
        if let Some(span) = candidate_span {
            write_snippet(output, program.source(), span, &candidate_label, 6, styles);
        }
        write_when_note(output, evaluation, &candidate.rule, styles);
        if let Some(source) = &candidate.source {
            write_metadata(output, 6, "source", format_args!("{source}"), styles);
        }

        if let Some(span) = override_location(program, by, &candidate.rule) {
            write_snippet(
                output,
                program.source(),
                span,
                &format!("rule `{by}` removed this candidate"),
                6,
                styles,
            );
        } else {
            write_metadata(output, 6, "overridden by", format_args!("{by}"), styles);
        }
        write_when_note(output, evaluation, by, styles);
    }
}

fn write_blocked_rules(
    output: &mut String,
    program: &CompiledProgram,
    evaluation: &Evaluation,
    explained_rules: &mut BTreeSet<String>,
    styles: Styles,
) {
    let blocked = evaluation
        .rules
        .iter()
        .filter(|trace| {
            matches!(
                trace.outcome,
                RuleOutcome::Blocked | RuleOutcome::Conflicted
            )
        })
        .collect::<Vec<_>>();
    if blocked.is_empty() {
        return;
    }

    output.push('\n');
    write_section(output, 4, format_args!("blocked rules"), styles);
    for trace in blocked {
        explained_rules.insert(normalize_name(&trace.rule));
        let effect_blocked = trace.condition == tess::value::TruthValue::True;
        let span = if effect_blocked {
            candidate_location(program, &trace.rule, &evaluation.result.decision).0
        } else {
            program.rule(&trace.rule).map(|rule| rule.condition.span)
        };
        let state = match trace.outcome {
            RuleOutcome::Conflicted => "conflicting evidence prevented this rule",
            RuleOutcome::Blocked if trace.missing.is_empty() => "this rule could not be resolved",
            RuleOutcome::Blocked => "this rule needs additional input",
            _ => unreachable!(),
        };
        if let Some(span) = span {
            write_snippet(
                output,
                program.source(),
                span,
                &format!("{state}: `{}`", trace.rule),
                6,
                styles,
            );
        }
        write_metadata(
            output,
            6,
            "when",
            format_args!("{} → {}", trace.condition_expression, trace.condition),
            styles,
        );
        if !trace.missing.is_empty() {
            write_metadata(
                output,
                6,
                "missing",
                format_args!("{}", trace.missing.join(", ")),
                styles,
            );
        }
        if let Some(source) = &trace.source {
            write_metadata(output, 6, "source", format_args!("{source}"), styles);
        }
    }
}

fn write_other_applied_rules(
    output: &mut String,
    program: &CompiledProgram,
    evaluation: &Evaluation,
    explained_rules: &BTreeSet<String>,
    styles: Styles,
) {
    let other = evaluation
        .rules
        .iter()
        .filter(|trace| {
            matches!(
                trace.outcome,
                RuleOutcome::Applied | RuleOutcome::OverrideApplied
            ) && !explained_rules.contains(&normalize_name(&trace.rule))
        })
        .collect::<Vec<_>>();
    if other.is_empty() {
        return;
    }
    output.push('\n');
    write_section(output, 4, format_args!("other applied rules"), styles);
    for trace in other {
        if let Some(rule) = program.rule(&trace.rule) {
            write_snippet(
                output,
                program.source(),
                rule.condition.span,
                &format!("condition evaluated to true for rule `{}`", trace.rule),
                6,
                styles,
            );
        }
        if let Some(source) = &trace.source {
            write_metadata(output, 6, "source", format_args!("{source}"), styles);
        }
    }
}

fn write_skipped_rules(
    output: &mut String,
    program: &CompiledProgram,
    evaluation: &Evaluation,
    styles: Styles,
) {
    let skipped = evaluation
        .rules
        .iter()
        .filter(|trace| trace.outcome == RuleOutcome::Skipped)
        .collect::<Vec<_>>();
    if skipped.is_empty() {
        return;
    }
    output.push('\n');
    write_section(
        output,
        4,
        format_args!(
            "not applicable: {}",
            counted(skipped.len(), "rule", "rules")
        ),
        styles,
    );
    for trace in skipped {
        if let Some(rule) = program.rule(&trace.rule) {
            write_snippet(
                output,
                program.source(),
                rule.condition.span,
                &format!(
                    "condition evaluated to false; rule `{}` was skipped",
                    trace.rule
                ),
                6,
                styles,
            );
        } else {
            write_metadata(
                output,
                6,
                "skipped",
                format_args!("`{}`: {} → false", trace.rule, trace.condition_expression),
                styles,
            );
        }
        if let Some(source) = &trace.source {
            write_metadata(output, 6, "source", format_args!("{source}"), styles);
        }
    }
}

fn write_help(output: &mut String, evaluation: &Evaluation, styles: Styles) {
    let help = match &evaluation.result.status {
        DecisionStatus::Undefined => Some(
            "add a rule covering this input, or use `zero or one` if no answer is valid".into(),
        ),
        DecisionStatus::Unknown { missing } => {
            Some(format!("provide the missing input: {}", missing.join(", ")))
        }
        DecisionStatus::Conflict { .. } => {
            Some("make the conditions mutually exclusive or add an explicit named override".into())
        }
        DecisionStatus::Resolved { .. } => None,
    };
    if let Some(help) = help {
        output.push('\n');
        write_metadata(output, 4, "help", format_args!("{help}"), styles);
    }
}

fn write_when_note(output: &mut String, evaluation: &Evaluation, rule_name: &str, styles: Styles) {
    if let Some(trace) = trace_for_rule(evaluation, rule_name) {
        write_metadata(
            output,
            6,
            "when",
            format_args!("{} → {}", trace.condition_expression, trace.condition),
            styles,
        );
    }
}

fn trace_for_rule<'a>(evaluation: &'a Evaluation, rule_name: &str) -> Option<&'a RuleTrace> {
    let rule_name = normalize_name(rule_name);
    evaluation
        .rules
        .iter()
        .find(|trace| normalize_name(&trace.rule) == rule_name)
}

fn candidate_location(
    program: &CompiledProgram,
    rule_name: &str,
    decision_name: &str,
) -> (Option<Span>, bool) {
    let Some(rule) = program.rule(rule_name) else {
        return (None, false);
    };
    let decision_name = normalize_name(decision_name);
    let spans = rule
        .effects
        .iter()
        .filter_map(|effect| match effect {
            Effect::Decide { decision, span, .. }
                if normalize_name(&decision.value) == decision_name =>
            {
                Some(*span)
            }
            Effect::Decide { .. } | Effect::Override { .. } => None,
        })
        .collect::<Vec<_>>();
    match spans.as_slice() {
        [span] => (Some(*span), true),
        _ => (Some(rule.condition.span), false),
    }
}

fn override_location(program: &CompiledProgram, by: &str, target: &str) -> Option<Span> {
    let rule = program.rule(by)?;
    let target = normalize_name(target);
    let spans = rule
        .effects
        .iter()
        .filter_map(|effect| match effect {
            Effect::Override { rule, span } if normalize_name(&rule.value) == target => Some(*span),
            Effect::Override { .. } | Effect::Decide { .. } => None,
        })
        .collect::<Vec<_>>();
    match spans.as_slice() {
        [span] => Some(*span),
        _ => None,
    }
}

fn write_snippet(
    output: &mut String,
    source: &SourceFile,
    span: Span,
    label: &str,
    indent: usize,
    styles: Styles,
) {
    let (line, column) = source.line_col(span.start);
    let text = source.line_text_at(span.start);
    let gutter = line.to_string().len().max(3);
    let source_name = display_source_name(source.name_at(span.start));

    write_indent(output, indent);
    styles.write(
        output,
        LOCATION_STYLE,
        format_args!("--> {source_name}:{line}:{column}"),
    );
    output.push('\n');

    write_indent(output, indent);
    styles.write(output, LOCATION_STYLE, format_args!("{:>gutter$} |", ""));
    output.push('\n');

    write_indent(output, indent);
    styles.write(output, LOCATION_STYLE, format_args!("{line:>gutter$} | "));
    output.push_str(text);
    output.push('\n');

    let selected_width = source
        .text
        .get(span.start..span.end)
        .and_then(|selected| selected.lines().next())
        .map_or(1, |selected| selected.chars().count().max(1));
    let remaining_width = text
        .chars()
        .count()
        .saturating_sub(column.saturating_sub(1))
        .max(1);
    let marker_width = selected_width.min(remaining_width);

    write_indent(output, indent);
    styles.write(
        output,
        LOCATION_STYLE,
        format_args!("{:>gutter$} | {}", "", " ".repeat(column.saturating_sub(1))),
    );
    styles.write(
        output,
        MARKER_STYLE,
        format_args!("{}", "^".repeat(marker_width)),
    );
    let _ = writeln!(output, " {label}");
}

fn write_section(output: &mut String, indent: usize, value: Arguments<'_>, styles: Styles) {
    write_indent(output, indent);
    styles.write(output, SECTION_STYLE, value);
    output.push('\n');
}

fn display_source_name(name: &str) -> Cow<'_, str> {
    let path = Path::new(name);
    if !path.is_absolute() {
        return Cow::Borrowed(name);
    }
    let Ok(current_directory) = std::env::current_dir() else {
        return Cow::Borrowed(name);
    };
    path.strip_prefix(current_directory)
        .ok()
        .and_then(Path::to_str)
        .map_or_else(|| Cow::Borrowed(name), Cow::Borrowed)
}

fn write_metadata(
    output: &mut String,
    indent: usize,
    label: &str,
    value: Arguments<'_>,
    styles: Styles,
) {
    write_indent(output, indent);
    styles.write(output, LABEL_STYLE, format_args!("{label}"));
    output.push_str(&" ".repeat(METADATA_VALUE_COLUMN.saturating_sub(label.len()).max(2)));
    let _ = output.write_fmt(value);
    output.push('\n');
}

fn write_indent(output: &mut String, indent: usize) {
    output.push_str(&" ".repeat(indent));
}

fn resolved_values(values: &[Value]) -> String {
    match values {
        [] => "[]".to_owned(),
        [value] => value.to_string(),
        values => format!("[{}]", display_values(values)),
    }
}

fn display_values(values: &[Value]) -> String {
    values
        .iter()
        .map(ToString::to_string)
        .collect::<Vec<_>>()
        .join(", ")
}

const fn cardinality_contract(cardinality: Cardinality) -> &'static str {
    match cardinality {
        Cardinality::ExactlyOne => "exactly one distinct value is required",
        Cardinality::ZeroOrOne => "zero or one distinct value is allowed",
        Cardinality::Many => "any number of distinct values is allowed",
    }
}

fn counted(count: usize, singular: &str, plural: &str) -> String {
    if count == 1 {
        format!("1 {singular}")
    } else {
        format!("{count} {plural}")
    }
}