const std = @import("std");

pub const TokenizeError = error{
    UnexpectedEnd,
    InvalidCharacter,
};

pub const TokenKind = enum {
    end,

    asterisk,
    asterisk_equal,
    brace_left,
    brace_right,
    bracket_left,
    bracket_right,
    colon,
    comma,
    dollar,
    dot,
    dot_dot,
    equal,
    equal_equal,
    greater_than,
    greater_than_equal,
    less_than,
    less_than_equal,
    minus,
    minus_equal,
    not_equal,
    paren_left,
    paren_right,
    percent,
    percent_equal,
    pipe,
    pipe_equal,
    plus,
    plus_equal,
    question,
    question_slash_slash,
    semicolon,
    slash,
    slash_equal,
    slash_slash,
    slash_slash_equal,

    identifier,
    number,
};

pub const Token = union(TokenKind) {
    end,

    asterisk,
    asterisk_equal,
    brace_left,
    brace_right,
    bracket_left,
    bracket_right,
    colon,
    comma,
    dollar,
    dot,
    dot_dot,
    equal,
    equal_equal,
    greater_than,
    greater_than_equal,
    less_than,
    less_than_equal,
    minus,
    minus_equal,
    not_equal,
    paren_left,
    paren_right,
    percent,
    percent_equal,
    pipe,
    pipe_equal,
    plus,
    plus_equal,
    question,
    question_slash_slash,
    semicolon,
    slash,
    slash_equal,
    slash_slash,
    slash_slash_equal,

    identifier: []const u8,
    number: i64,

    pub fn kind(self: @This()) TokenKind {
        return self;
    }
};

fn peekByte(reader: *std.Io.Reader) error{ReadFailed}!?u8 {
    return reader.peekByte() catch |err| switch (err) {
        error.EndOfStream => null,
        error.ReadFailed => error.ReadFailed,
    };
}

fn takeByteIf(reader: *std.Io.Reader, expected: u8) error{ReadFailed}!bool {
    if (try peekByte(reader) == expected) {
        reader.toss(1);
        return true;
    } else {
        return false;
    }
}

fn isIdentifierStart(c: u8) bool {
    return std.ascii.isAlphabetic(c) or c == '_';
}

fn isIdentifierContinue(c: u8) bool {
    return std.ascii.isAlphanumeric(c) or c == '_';
}

fn tokenizeIdentifier(allocator: std.mem.Allocator, reader: *std.Io.Reader, first: u8) ![]const u8 {
    var buffer = try std.array_list.Aligned(u8, null).initCapacity(allocator, 16);
    try buffer.append(allocator, first);

    while (true) {
        // Read an identifier.
        while (try peekByte(reader)) |c| {
            if (isIdentifierContinue(c)) {
                try buffer.append(allocator, c);
                reader.toss(1);
            } else {
                break;
            }
        }

        // Check namespaced identifier (e.g., "foo::bar").
        const lookahead = reader.peek(3) catch |err| switch (err) {
            error.EndOfStream => break,
            error.ReadFailed => return error.ReadFailed,
        };
        if (lookahead[0] == ':' and lookahead[1] == ':' and isIdentifierStart(lookahead[2])) {
            try buffer.append(allocator, ':');
            try buffer.append(allocator, ':');
            try buffer.append(allocator, lookahead[2]);
            reader.toss(3);
            continue;
        } else {
            break;
        }
    }

    return buffer.toOwnedSlice(allocator);
}

pub fn tokenize(allocator: std.mem.Allocator, reader: *std.Io.Reader) ![]Token {
    var tokens = try std.array_list.Aligned(Token, null).initCapacity(allocator, 16);

    while (true) {
        const c = reader.takeByte() catch |err| switch (err) {
            error.EndOfStream => break,
            error.ReadFailed => return error.ReadFailed,
        };
        const token: Token = switch (c) {
            ' ', '\t', '\n', '\r' => continue,
            '$' => .dollar,
            '%' => if (try takeByteIf(reader, '=')) .percent_equal else .percent,
            '(' => .paren_left,
            ')' => .paren_right,
            '*' => if (try takeByteIf(reader, '=')) .asterisk_equal else .asterisk,
            '+' => if (try takeByteIf(reader, '=')) .plus_equal else .plus,
            ',' => .comma,
            '-' => if (try takeByteIf(reader, '=')) .minus_equal else .minus,
            '.' => if (try takeByteIf(reader, '.')) .dot_dot else .dot,
            '/' => if (try takeByteIf(reader, '/'))
                if (try takeByteIf(reader, '=')) .slash_slash_equal else .slash_slash
            else if (try takeByteIf(reader, '='))
                .slash_equal
            else
                .slash,
            ':' => .colon,
            ';' => .semicolon,
            '<' => if (try takeByteIf(reader, '=')) .less_than_equal else .less_than,
            '=' => if (try takeByteIf(reader, '=')) .equal_equal else .equal,
            '>' => if (try takeByteIf(reader, '=')) .greater_than_equal else .greater_than,
            '!' => if (try takeByteIf(reader, '=')) .not_equal else return error.InvalidCharacter,
            '?' => if (try takeByteIf(reader, '/'))
                if (try takeByteIf(reader, '/')) .question_slash_slash else return error.InvalidCharacter
            else
                .question,
            '[' => .bracket_left,
            ']' => .bracket_right,
            '{' => .brace_left,
            '|' => if (try takeByteIf(reader, '=')) .pipe_equal else .pipe,
            '}' => .brace_right,
            else => if (std.ascii.isDigit(c))
                .{ .number = (c - '0') }
            else if (isIdentifierStart(c))
                .{ .identifier = try tokenizeIdentifier(allocator, reader, c) }
            else
                return error.InvalidCharacter,
        };
        try tokens.append(allocator, token);
    }

    if (tokens.items.len == 0) {
        return error.UnexpectedEnd;
    }

    try tokens.append(allocator, .end);
    return tokens.toOwnedSlice(allocator);
}

test "tokenize symbols" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed(
        \\* *= { } [ ] : , $ . .. = == > >= < <= - -= != ( ) % %=
        \\| |= + += ? ?// ; / /= // //=
    );
    const tokens = try tokenize(allocator.allocator(), &reader);

    const expected = [_]Token{
        .asterisk,
        .asterisk_equal,
        .brace_left,
        .brace_right,
        .bracket_left,
        .bracket_right,
        .colon,
        .comma,
        .dollar,
        .dot,
        .dot_dot,
        .equal,
        .equal_equal,
        .greater_than,
        .greater_than_equal,
        .less_than,
        .less_than_equal,
        .minus,
        .minus_equal,
        .not_equal,
        .paren_left,
        .paren_right,
        .percent,
        .percent_equal,
        .pipe,
        .pipe_equal,
        .plus,
        .plus_equal,
        .question,
        .question_slash_slash,
        .semicolon,
        .slash,
        .slash_equal,
        .slash_slash,
        .slash_slash_equal,
        .end,
    };
    try std.testing.expectEqual(expected.len, tokens.len);
    for (expected, tokens) |e, t| {
        try std.testing.expectEqual(e, t);
    }
}

test "tokenize number" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed("5");
    const tokens = try tokenize(allocator.allocator(), &reader);

    try std.testing.expectEqual(2, tokens.len);
    try std.testing.expectEqual(Token{ .number = 5 }, tokens[0]);
    try std.testing.expectEqual(.end, tokens[1]);
}

test "tokenize array index" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed(".[0]");
    const tokens = try tokenize(allocator.allocator(), &reader);

    try std.testing.expectEqual(5, tokens.len);
    try std.testing.expectEqual(.dot, tokens[0]);
    try std.testing.expectEqual(.bracket_left, tokens[1]);
    try std.testing.expectEqual(Token{ .number = 0 }, tokens[2]);
    try std.testing.expectEqual(.bracket_right, tokens[3]);
    try std.testing.expectEqual(.end, tokens[4]);
}

test "tokenize empty input returns error" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed("");
    const result = tokenize(allocator.allocator(), &reader);

    try std.testing.expectError(error.UnexpectedEnd, result);
}

test "tokenize invalid character returns error" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed("`");
    const result = tokenize(allocator.allocator(), &reader);

    try std.testing.expectError(error.InvalidCharacter, result);
}

test "tokenize identifiers" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed("foo _foo foo2");
    const tokens = try tokenize(allocator.allocator(), &reader);

    try std.testing.expectEqual(4, tokens.len);
    try std.testing.expectEqualStrings("foo", tokens[0].identifier);
    try std.testing.expectEqualStrings("_foo", tokens[1].identifier);
    try std.testing.expectEqualStrings("foo2", tokens[2].identifier);
    try std.testing.expectEqual(.end, tokens[3]);
}

test "tokenize namespaced identifier" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed("foo::bar foo::bar::baz");
    const tokens = try tokenize(allocator.allocator(), &reader);

    try std.testing.expectEqual(3, tokens.len);
    try std.testing.expectEqualStrings("foo::bar", tokens[0].identifier);
    try std.testing.expectEqualStrings("foo::bar::baz", tokens[1].identifier);
    try std.testing.expectEqual(.end, tokens[2]);
}

test "tokenize identifier followed by colon" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed("foo:bar");
    const tokens = try tokenize(allocator.allocator(), &reader);

    try std.testing.expectEqual(4, tokens.len);
    try std.testing.expectEqualStrings("foo", tokens[0].identifier);
    try std.testing.expectEqual(.colon, tokens[1]);
    try std.testing.expectEqualStrings("bar", tokens[2].identifier);
    try std.testing.expectEqual(.end, tokens[3]);
}

test "tokenize identifier in complex query" {
    var allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer allocator.deinit();

    var reader = std.Io.Reader.fixed(".foo | bar::baz");
    const tokens = try tokenize(allocator.allocator(), &reader);

    try std.testing.expectEqual(5, tokens.len);
    try std.testing.expectEqual(.dot, tokens[0]);
    try std.testing.expectEqualStrings("foo", tokens[1].identifier);
    try std.testing.expectEqual(.pipe, tokens[2]);
    try std.testing.expectEqualStrings("bar::baz", tokens[3].identifier);
    try std.testing.expectEqual(.end, tokens[4]);
}