// SPDX-License-Identifier: MIT
// Copyright (c) 2015-2020 Zig Contributors
// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
// The MIT license requires this copyright notice to be included in all copies
// and substantial portions of the software.
const std = @import("std");
const assert = std.debug.assert;
const Allocator = std.mem.Allocator;
const ast = @import("ast.zig");
const Node = ast.Node;
const Tree = ast.Tree;
const AstError = ast.Error;
const TokenIndex = ast.TokenIndex;
const NodeIndex = ast.NodeIndex;

const lexer = @import("tokenizer.zig");
const Token = lexer.Token;
const Tokenizer = lexer.Tokenizer;

pub const Error = error{ParseError} || Allocator.Error;

/// Result should be freed with tree.deinit() when there are
/// no more references to any of the tokens or nodes.
pub fn parse(gpa: *Allocator, source: []const u8) Allocator.Error!*Tree {

    var token_ids = std.ArrayList(Token.Id).init(gpa);
    defer token_ids.deinit();
    var token_locs = std.ArrayList(Token.Loc).init(gpa);
    defer token_locs.deinit();

    // Before starting, just check that we have about enough memory.
    const estimated_token_count = source.len / 8;
    try token_ids.ensureCapacity(estimated_token_count);
    try token_locs.ensureCapacity(estimated_token_count);

    // Use the lexer to get all the tokens from the source code.
    var tokenizer = Tokenizer.init(source);
    while (true) {
        const token = tokenizer.next();
        try token_ids.append(token.id);
        try token_locs.append(token.loc);
        if (token.id == .Eof) break;
    }

    // Create a Parser structure.
    var parser: Parser = .{
        .source = source,                           // Source code.
        .arena = std.heap.ArenaAllocator.init(gpa), // Arena allocator.
        .gpa = gpa,                                 // General Purpose Allocator.
        .token_ids = token_ids.items,               // IDs of the tokens.
        .token_locs = token_locs.items,             // Location of the tokens.
        .errors = .{},                              // List of errors in our parsing.
        .tok_i = 0,                                 // Index of current token being analyzed.
        .indent = 0,                                // Indentation for debug.
    };
    defer parser.errors.deinit(gpa);
    errdefer parser.arena.deinit();

    // Ignore the first line comments from our code.
    while (token_ids.items[parser.tok_i] == .LineComment) parser.tok_i += 1;

    // Perform parsing, called once.
    const root_node = try parser.parseRoot();

    // Create a parsing Tree, with the nodes parsed early on.
    // toOwnedSlice: free the memory and return the list. Arrays are empty,
    // allocator can be free, arrays are owned by a different allocator.
    const tree = try parser.arena.allocator.create(Tree);
    tree.* = .{
        .gpa = gpa,
        .source = source,
        .token_ids = token_ids.toOwnedSlice(),
        .token_locs = token_locs.toOwnedSlice(),
        .errors = parser.errors.toOwnedSlice(gpa),
        .root_node = root_node,
        .arena = parser.arena.state,
    };
    return tree;
}

const Assignment = struct {
    id_attribute: TokenIndex,
    id_value:     TokenIndex,
};

/// Represents in-progress parsing, will be converted to an ast.Tree after completion.
const Parser = struct {
    arena: std.heap.ArenaAllocator,
    gpa: *Allocator,
    source: []const u8,
    token_ids: []const Token.Id,
    token_locs: []const Token.Loc,
    tok_i: TokenIndex,
    errors: std.ArrayListUnmanaged(AstError),
    indent: u16, // Indentation for debug.

    /// Root <- skip ContainerMembers eof
    fn parseRoot(p: *Parser) Allocator.Error!*Node.Root {
        // Parse declarations.
        const decls = try parseContainerMembers(p, true);
        defer p.gpa.free(decls);

        // parseContainerMembers will try to skip as much
        // invalid tokens as it can so this can only be the EOF
        // eatToken returns next token or null (if current token id isn't parameter).
        // If current token is .Eof, next token is actually the first.
        const eof_token = p.eatToken(.Eof).?;

        // Nb of declarations becomes an ast.NodeIndex integer variable (usize).
        const decls_len = @intCast(NodeIndex, decls.len);
        const node = try Node.Root.create(&p.arena.allocator, decls_len, eof_token);
        // std.mem.copy: T, dest, src
        std.mem.copy(*Node, node.decls(), decls);

        return node; // Root node.
    }

    /// ContainerMembers
    ///     <- TestDecl ContainerMembers
    ///      / TopLevelComptime ContainerMembers
    ///      / KEYWORD_pub? TopLevelDecl ContainerMembers
    ///      / ContainerField COMMA ContainerMembers
    ///      / ContainerField
    ///      /
    // parseContainerMembers: actual parsing code starts here.
    fn parseContainerMembers(p: *Parser, top_level: bool) ![]*Node {
        std.debug.print("parseContainerMembers: is top? {}\n", .{top_level});
        // list: all nodes in the ast.
        var list = std.ArrayList(*Node).init(p.gpa);
        defer list.deinit();

        // field_state: union of enum.
        // Tagged union: eligible to use in switch expressions and coerce their value.
        // Example: switch (some_tagged_union) { SomeType => |value| print("{}\n", value); }
        // If a '*' is placed before the variable name, it's a pointer to the value inside
        // the tagged union.
        // Example: switch (some_tagged_union) { SomeType => |*value| value.* += 1; }
        // @TagType can be used to get the right enum type.
        var field_state: union(enum) {
            /// no fields have been seen
            none,
            /// currently parsing fields
            seen,
            /// saw fields and then a declaration after them.
            /// payload is first token of previous declaration.
            end: TokenIndex, // TokenIndex is defined as usize in std.zig.ast.
            /// there was a declaration between fields, don't report more errors
            err,
        } = .none;

        // Start     => Requires Thing => End
        // Requires  => require StringLiteral Requires | nil
        // Thing     => Definition Thing | ClassHdr Thing | nil

        // Definition => define Identifier ClassHdrSimple

        // ClassHdr  => ClassHdrFull LBrace ClassCon RBrace |
        //              ClassHdrSimple LBrace ClassCon RBrace
        // ClassHdrSimple: a class without identifier,
        //                 for a definition or when this won't be used later.
        // ClassHdrSimple => Identifier
        // ClassHdrFull: a class with an identifier.
        // ClassHdrFull   => Identifier LParen Identifier RParen
        // ClassCon  => ClassHdr | statement ClassCon | nil

        // Property keyword adds a new property to the definition,
        // without it, default property values can be changed.

        // statement => Keyword_property Identifier Identifier Colon value
        // value     => StringLiteral | Keyword_null | IntegerLiteral | FloatLiteral

        // True start of parsing.
        while (true) {
            const token = p.nextToken();
            switch (p.token_ids[token]) {
                .Keyword_require => {
                    p.putBackToken(token);
                    // TODO: read file required and parse its content.
                    p.parseRequire();
                },

                .Keyword_define => {
                    p.putBackToken(token);
                    // TODO: definitions.
                    p.parseDefine() catch |err| switch(err) {
                        // Propagate memory errors.
                        error.OutOfMemory => { return (error.OutOfMemory); },
                        error.ParseError => {
                             continue;
                        }
                    }; // |normal_value| { stuff; to; do; }
                },

                .Identifier => {
                    // Identifier => on top level this means a class.
                    p.putBackToken(token);
                    p.parseClass() catch |err| switch(err) {
                        // Propagate memory errors.
                        error.OutOfMemory => { return (error.OutOfMemory); },
                        error.ParseError => {
                             p.say("we catched a ParseError on token: {}\n"
                                 , .{p.giveTokenContent(p.tok_i)});
                             continue;
                        }
                    }; // |normal_value| { stuff; to; do; }
                },

                .Eof => {
                    p.putBackToken(token);
                    break;
                },

                else => {
                    std.debug.print("token: {}\n", .{p.token_ids[token]});
                    continue;
                },
            }
        }

        return list.toOwnedSlice();
    }

    fn say(p: *Parser, comptime fmt: []const u8, args: anytype) void {
        var i = p.indent;
        while (i > 0) {
            std.debug.print("\t", .{});
            i-=1;
        }
        std.debug.print(fmt, args);
    }

    // TODO: require "file"
    //       file should be read, parsed and a loop detection should take place.
    fn parseRequire(p: *Parser) void {
        const require_token = p.eatToken(.Keyword_require);
        const file_to_read = p.eatToken(.StringLiteral);
        std.debug.print("TODO: file required: {}\n", .{file_to_read});
    }

    // TODO: class definition (inheritance).
    // fn parseDefine(p: *Parser) !?Definition {
    fn parseDefine(p: *Parser) !void {
        const define_token = p.eatToken(.Keyword_define);
        const new_class_name = p.eatToken(.Identifier);
        const parent_class_name = p.eatToken(.Identifier);
        std.debug.print("TODO: class inheritance: {} < {}\n",
            .{new_class_name, parent_class_name});
        try p.parseClass();
        // TODO: get the old class definition,
        //       create a new definition,
        //       then add old and new properties and children to it.
    }

    // TODO: class definition (inheritance).
    fn parseClass(p: *Parser) !void {
        if (p.indent > 0) {
            p.say("INNER parseClass\n", .{});
        }
        else {
            p.say("TOP parseClass\n", .{});
        }
        p.indent += 1;
        defer { p.indent -= 1; }

        const class_name = p.eatToken(.Identifier);
        if (class_name == null) {
             return;
        }
        // Either simple or full header.
        const identifier: ?[] const u8 = try p.parseFullClassHeader();

        p.say("TODO: read class: {}", .{p.giveTokenContent(class_name.?)});

        if (identifier) |id| {
            std.debug.print(", id: {}\n", .{id});
        }
        else {
            std.debug.print("\n", .{});
        }

        // Starting the class.
        // const lbrace = p.nextToken();
        const ignored = try p.expectToken(.LBrace);

        while (true) {
            // TODO: parsing class content.
            // TODO: loop over this.
            const token = p.nextToken();
            p.say("(top class reading) reading token: {}\n", .{p.giveTokenContent(token)});
            switch (p.token_ids[token]) {

                .Identifier => {
                    const following = p.nextToken();
                    switch (p.token_ids[following]) {
                        // Class header (with or without id).
                        .LParen,
                        .LBrace => {
                            p.putBackToken(following);
                            p.putBackToken(token);
                            p.say("reading a new class\n", .{});
                            // WARNING: RECURSION: this may cause errors.
                            const res = p.parseClass();
                            continue;
                        },

                        .Colon => {
                            p.putBackToken(following);
                            p.putBackToken(token);
                            const assignment = try p.parseAssignment();
                            p.say("redefining an attribute {} => {}\n"
                                , .{  p.giveTokenContent(assignment.id_attribute)
                                    , p.giveTokenContent(assignment.id_value)});
                        },

                        else => {
                            // Wasn't expected.
                            // Couln't understand what was in this class.
                            p.say("did not understand {} then {}\n"
                                , .{  p.giveTokenContent(token)
                                    , p.giveTokenContent(following)});
                            p.putBackToken(following);
                            p.putBackToken(token);
                            break;
                        }
                    }
                },

                .Keyword_property => {
                    p.say("Reading a property\n", .{});
                    p.putBackToken(token);
                    try p.parseProperty();
                    p.say("Done reading a property\n", .{});
                    continue;
                },

                .LBrace => {
                    p.say("Reading a LBrace\n", .{});
                },

                .RBrace => {
                    p.say("Reading a RBrace\n", .{});
                    p.putBackToken(token);
                    break;
                },

                else => {
                    p.putBackToken(token);
                    p.say("reading {} in a class, backing up\n"
                        , .{p.giveTokenContent(token)});
                    break;
                }
            }
        }

        // Class definition or instance ends with a RBrace.
        const end_of_class = try p.expectToken(.RBrace);
    }

    fn parseProperty(p: *Parser) !void {
        const property = try p.expectToken(.Keyword_property);
        const class_name = try p.expectToken(.Identifier);
        const attribute_name = try p.expectToken(.Identifier);
        const colon = try p.expectToken(.Colon);
        const id_value = p.nextToken();
        switch (p.token_ids[id_value]) {
             .Keyword_null,
             .StringLiteral,
             .IntegerLiteral,
             .FloatLiteral => {
                   p.say("property: {} {} = {}\n"
                       , .{p.giveTokenContent(class_name), p.giveTokenContent(attribute_name), p.giveTokenContent(id_value)});
                   return ;
                   // return Assignment{.id_attribute = ia, .id_value = id_value};
             },
             else => {
                  return error.ParseError;
             }
        }
    }


    // statement => Keyword_property Identifier Identifier Colon value
    // value     => StringLiteral | Keyword_null | IntegerLiteral | FloatLiteral
    fn parseAssignment(p: *Parser) !Assignment {
        const id_attribute = p.eatToken(.Identifier);
        const ignored = try p.expectToken(.Colon);
        const id_value = p.nextToken();
        switch (p.token_ids[id_value]) {
             .Keyword_null,
             .StringLiteral,
             .IntegerLiteral,
             .FloatLiteral => {
                  if (id_attribute) |ia| {
                       return Assignment{.id_attribute = ia, .id_value = id_value};
                  }
             },
             else => {
                  return error.ParseError;
             }
        }
        return error.ParseError;
    }

    fn parseFullClassHeader(p: *Parser) !?[]const u8 {
        if (p.eatToken(.LParen) == null)
            return null;

        // Once we know this is a full header, an identifier then a right
        // parenthesis are expected, and should trigger a parsing error if not there.
        const identifier = try p.expectToken(.Identifier);
        const blah = try p.expectToken(.RParen);

        return p.giveTokenContent(identifier);
    }

    // fn parseStatement(p: *Parser) Error!?*Node {
    // }


    fn eatToken(p: *Parser, id: Token.Id) ?TokenIndex {
        return if (p.token_ids[p.tok_i] == id) p.nextToken() else null;
    }

    // expectToken: either returns the token or an error.
    fn expectToken(p: *Parser, id: Token.Id) Error!TokenIndex {
        return (try p.expectTokenRecoverable(id)) orelse error.ParseError;
    }

    // expectTokenRecoverable: either returns the token or null if not the one expected.
    //                         Also, appends the error inside p.errors.
    fn expectTokenRecoverable(p: *Parser, id: Token.Id) !?TokenIndex {
        const token = p.nextToken();
        if (p.token_ids[token] != id) {
            try p.errors.append(p.gpa, .{
                .ExpectedToken = .{ .token = token, .expected_id = id },
            });
            // go back so that we can recover properly
            p.putBackToken(token);
            return null;
        }
        return token;
    }

    // nextToken: provide the TokenIndex of the current token, but increases the tok_i
    //            inside the Parser structure.
    fn nextToken(p: *Parser) TokenIndex {
        const result = p.tok_i;
        p.tok_i += 1;
        assert(p.token_ids[result] != .LineComment);
        if (p.tok_i >= p.token_ids.len) return result;

        while (true) {
            if (p.token_ids[p.tok_i] != .LineComment) return result;
            p.tok_i += 1;
        }
    }

    // putBackToken: come back one token (except for comment lines which are ignored).
    // Example: we have a function searching for a declaration,
    //          the function read a token "my-variable" so it returns after putting back the token.
    //          Caller now have the start of a declaration in its parsing structure.
    fn putBackToken(p: *Parser, putting_back: TokenIndex) void {
        while (p.tok_i > 0) {
            p.tok_i -= 1;
            if (p.token_ids[p.tok_i] == .LineComment) continue;
            assert(putting_back == p.tok_i);
            return;
        }
    }

    /// TODO Delete this function. I don't like the inversion of control.
    fn expectNode(
        p: *Parser,
        parseFn: NodeParseFn,
        /// if parsing fails
        err: AstError,
    ) Error!*Node {
        return (try p.expectNodeRecoverable(parseFn, err)) orelse return error.ParseError;
    }

    /// TODO Delete this function. I don't like the inversion of control.
    fn expectNodeRecoverable(
        p: *Parser,
        parseFn: NodeParseFn,
        /// if parsing fails
        err: AstError,
    ) !?*Node {
        return (try parseFn(p)) orelse {
            try p.errors.append(p.gpa, err);
            return null;
        };
    }

    // WARNING: VALID token identifier expected.
    fn giveTokenContent(p: *Parser, id: TokenIndex) []const u8 {
        const loc = p.token_locs[id];
        return p.source[loc.start..loc.end];
    }
};

fn ParseFn(comptime T: type) type {
    return fn (p: *Parser) Error!T;
}

test "std.zig.parser" {
    _ = @import("parser_test.zig");
}