// SPDX-License-Identifier: LGPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

module Crafter.Debug:Condition_impl;
import std;
import Crafter.Debug.Core;
import :Condition;

using namespace Crafter::Debug;

struct Condition::Node {
    enum class Op : std::uint8_t {
        Or,
        And,
        Not,
        Compare,
        Truthy,
    };
    enum class Comparison : std::uint8_t {
        Equal,
        NotEqual,
        Less,
        LessEqual,
        Greater,
        GreaterEqual,
    };
    struct Operand {
        // A path to resolve, or else a literal.
        std::string path;
        Value literal;
    };

    Op op = Op::Truthy;
    Comparison comparison = Comparison::Equal;
    std::shared_ptr<const Node> left;
    std::shared_ptr<const Node> right;
    Operand a;
    Operand b;
    // The text this node was parsed from, for errors.
    std::string source;
};

namespace {
    using Node = Condition::Node;
    using Outcome = std::expected<bool, std::string>;

    // Nesting deeper than this is refused rather than recursed into.
    constexpr std::uint32_t maxDepth = 64;

    bool PathStart(char c) {
        return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_';
    }

    bool PathChar(char c) {
        return PathStart(c) || (c >= '0' && c <= '9') || c == '-';
    }

    std::string_view ComparisonText(Node::Comparison comparison) {
        switch (comparison) {
        case Node::Comparison::Equal: return "==";
        case Node::Comparison::NotEqual: return "!=";
        case Node::Comparison::Less: return "<";
        case Node::Comparison::LessEqual: return "<=";
        case Node::Comparison::Greater: return ">";
        case Node::Comparison::GreaterEqual: return ">=";
        }
        return "?";
    }

    class Parser {
    public:
        Parser(std::string_view text, std::vector<std::string>& paths) : text(text), paths(paths) {}

        std::shared_ptr<const Node> Document() {
            auto node = Or(0);
            Skip();
            if (!AtEnd()) Fail("unexpected text; join conditions with && or ||");
            return node;
        }

    private:
        std::string_view text;
        std::vector<std::string>& paths;
        std::size_t position = 0;

        [[noreturn]] void Fail(std::string_view what) const {
            throw ConditionError(position, what);
        }

        bool AtEnd() const {
            return position >= text.size();
        }

        void Skip() {
            while (!AtEnd() && (text[position] == ' ' || text[position] == '\t')) ++position;
        }

        bool Match(std::string_view token) {
            Skip();
            if (text.substr(position, token.size()) != token) return false;
            position += token.size();
            return true;
        }

        std::shared_ptr<const Node> Combine(Node::Op op, std::shared_ptr<const Node> left, std::shared_ptr<const Node> right) {
            auto node = std::make_shared<Node>();
            node->op = op;
            node->left = std::move(left);
            node->right = std::move(right);
            return node;
        }

        std::shared_ptr<const Node> Or(std::uint32_t depth) {
            auto left = And(depth);
            while (Match("||")) left = Combine(Node::Op::Or, std::move(left), And(depth));
            return left;
        }

        std::shared_ptr<const Node> And(std::uint32_t depth) {
            auto left = Not(depth);
            while (Match("&&")) left = Combine(Node::Op::And, std::move(left), Not(depth));
            return left;
        }

        std::shared_ptr<const Node> Not(std::uint32_t depth) {
            if (depth > maxDepth) Fail("nested too deeply");
            Skip();
            if (!AtEnd() && text[position] == '!' && text.substr(position, 2) != "!=") {
                ++position;
                return Combine(Node::Op::Not, Not(depth + 1), nullptr);
            }
            return Primary(depth);
        }

        std::shared_ptr<const Node> Primary(std::uint32_t depth) {
            if (Match("(")) {
                auto inner = Or(depth + 1);
                if (!Match(")")) Fail("expected ')'");
                return inner;
            }
            Skip();
            const std::size_t start = position;
            auto node = std::make_shared<Node>();
            node->a = Operand();
            if (const auto comparison = ComparisonOperator()) {
                node->op = Node::Op::Compare;
                node->comparison = *comparison;
                node->b = Operand();
            } else {
                node->op = Node::Op::Truthy;
            }
            node->source = std::string(text.substr(start, position - start));
            while (!node->source.empty() && node->source.back() == ' ') node->source.pop_back();
            return node;
        }

        std::optional<Node::Comparison> ComparisonOperator() {
            Skip();
            if (Match("==")) return Node::Comparison::Equal;
            if (Match("!=")) return Node::Comparison::NotEqual;
            if (Match("<=")) return Node::Comparison::LessEqual;
            if (Match(">=")) return Node::Comparison::GreaterEqual;
            if (Match("<")) return Node::Comparison::Less;
            if (Match(">")) return Node::Comparison::Greater;
            if (!AtEnd() && text[position] == '=') Fail("use == to compare");
            return std::nullopt;
        }

        Node::Operand Operand() {
            Skip();
            if (AtEnd()) Fail("expected a path or a value");
            const char c = text[position];
            if (c == '"' || c == '\'') return { {}, Value(String(c)) };
            if ((c >= '0' && c <= '9') || c == '-') return { {}, Number() };
            if (!PathStart(c)) Fail("expected a path or a value");
            const std::size_t start = position;
            while (!AtEnd()) {
                const char next = text[position];
                if (PathChar(next) || next == '.') {
                    ++position;
                } else if (next == '[') {
                    const std::size_t close = text.find(']', position);
                    if (close == std::string_view::npos) Fail("expected ']'");
                    position = close + 1;
                } else {
                    break;
                }
            }
            const std::string_view word = text.substr(start, position - start);
            if (word == "true") return { {}, Value(true) };
            if (word == "false") return { {}, Value(false) };
            if (word == "null") return { {}, Value() };
            paths.emplace_back(word);
            return { std::string(word), Value() };
        }

        std::string String(char quote) {
            ++position;
            std::string out;
            while (true) {
                if (AtEnd()) Fail("unterminated string");
                const char c = text[position++];
                if (c == quote) return out;
                if (c == '\\' && !AtEnd()) {
                    out += text[position++];
                    continue;
                }
                out += c;
            }
        }

        Value Number() {
            const std::size_t start = position;
            if (text[position] == '-') ++position;
            while (!AtEnd() && ((text[position] >= '0' && text[position] <= '9') || text[position] == '.' || text[position] == 'e' || text[position] == 'E'
                                || ((text[position] == '+' || text[position] == '-') && (text[position - 1] == 'e' || text[position - 1] == 'E')))) {
                ++position;
            }
            try {
                return Value::Parse(text.substr(start, position - start));
            } catch (const ParseError&) {
                position = start;
                Fail("malformed number");
            }
        }
    };

    std::expected<Value, std::string> Load(const Node::Operand& operand, const Condition::Resolver& resolve) {
        if (operand.path.empty()) return operand.literal;
        return resolve(operand.path);
    }

    bool Truthy(const Value& value) {
        switch (value.Kind()) {
        case ValueKind::Null: return false;
        case ValueKind::Bool: return value.AsBool();
        case ValueKind::Integer: return value.AsInteger() != 0;
        case ValueKind::Number: return value.AsNumber() != 0;
        case ValueKind::String: return !value.AsString().empty();
        case ValueKind::Array:
        case ValueKind::Object: return value.Size() != 0;
        }
        return false;
    }

    template<class T>
    bool Ordered(Node::Comparison comparison, const T& a, const T& b) {
        switch (comparison) {
        case Node::Comparison::Less: return a < b;
        case Node::Comparison::LessEqual: return a <= b;
        case Node::Comparison::Greater: return a > b;
        case Node::Comparison::GreaterEqual: return a >= b;
        default: return false;
        }
    }

    Outcome Compare(const Node& node, const Value& a, const Value& b) {
        if (node.comparison == Node::Comparison::Equal) return a == b;
        if (node.comparison == Node::Comparison::NotEqual) return !(a == b);
        if (a.IsInteger() && b.IsInteger()) return Ordered(node.comparison, a.AsInteger(), b.AsInteger());
        if (a.IsNumber() && b.IsNumber()) return Ordered(node.comparison, a.AsNumber(), b.AsNumber());
        if (a.IsString() && b.IsString()) return Ordered(node.comparison, a.AsString(), b.AsString());
        return std::unexpected(std::format("cannot order {} against {} in '{}': {} needs two numbers or two strings", a.Dump(), b.Dump(), node.source,
                                           ComparisonText(node.comparison)));
    }

    Outcome Evaluate(const Node& node, const Condition::Resolver& resolve) {
        switch (node.op) {
        case Node::Op::Or: {
            const Outcome left = Evaluate(*node.left, resolve);
            if (!left || *left) return left;
            return Evaluate(*node.right, resolve);
        }
        case Node::Op::And: {
            const Outcome left = Evaluate(*node.left, resolve);
            if (!left || !*left) return left;
            return Evaluate(*node.right, resolve);
        }
        case Node::Op::Not: {
            const Outcome inner = Evaluate(*node.left, resolve);
            if (!inner) return inner;
            return !*inner;
        }
        case Node::Op::Truthy: {
            const auto value = Load(node.a, resolve);
            if (!value) return std::unexpected(value.error());
            return Truthy(*value);
        }
        case Node::Op::Compare: {
            const auto a = Load(node.a, resolve);
            if (!a) return std::unexpected(a.error());
            const auto b = Load(node.b, resolve);
            if (!b) return std::unexpected(b.error());
            return Compare(node, *a, *b);
        }
        }
        return std::unexpected("unknown node");
    }
}

ConditionError::ConditionError(std::size_t offset, std::string_view what)
    : std::runtime_error(std::format("{} at offset {}", what, offset)), offset(offset) {}

Condition Condition::Parse(std::string_view text) {
    Condition condition;
    condition.text = std::string(text);
    Parser parser(condition.text, condition.paths);
    condition.root = parser.Document();
    return condition;
}

std::expected<bool, std::string> Condition::Evaluate(const Resolver& resolve) const {
    return ::Evaluate(*root, resolve);
}