//! Bencoding
//! See specification here https://wiki.theory.org/BitTorrentSpecification#Bencoding

const std = @import("std");

pub const Error = error.Malformatted || std.io.AnyReader.Error;

// All content is owned by the BValue and must be freed with deinit.
pub const BValue = union(enum) {
    string: []const u8,
    int: i64,
    list: std.ArrayList(BValue),
    dict: std.StringArrayHashMap(BValue),

    pub fn deinit(self: *BValue, a: std.mem.Allocator) void {
        switch (self.*) {
            .string => |s| {
                a.free(s);
            },
            .list => |*l| {
                for (l.items) |*i| {
                    i.deinit(a);
                }
                l.deinit();
            },
            .dict => |*d| {
                var it = d.iterator();
                while (it.next()) |entry| {
                    a.free(entry.key_ptr.*);
                    entry.value_ptr.*.deinit(a);
                }
                d.deinit();
            },
            .int => {},
        }
    }
};

pub fn bdecodeBuf(a: std.mem.Allocator, buf: []const u8) !BValue {
    var fbs = std.io.fixedBufferStream(buf);
    return try bdecode(a, fbs.reader());
}

pub fn bdecode(a: std.mem.Allocator, base_reader: anytype) anyerror!BValue {
    var reader = PeekStream.init(base_reader.any());
    return bdecodeInner(a, &reader, 0);
}

const PeekStream = std.io.PeekStream(.{ .Static = 1 }, std.io.AnyReader);

// Note: uses defined types only to avoid trying to recursively evaulate this function
// at compile time, otherwise we run into https://github.com/ziglang/zig/issues/13724
fn bdecodeInner(a: std.mem.Allocator, peekStream: *PeekStream, depth: u32) !BValue {
    if (depth > 100) {
        // TODO diagnostic...
        return error.Malformatted;
    }
    var reader = peekStream.reader();
    var byte = try reader.readByte();
    if (std.ascii.isDigit(byte)) {
        try peekStream.putBackByte(byte);
        return .{ .string = try readString(a, peekStream) };
    } else {
        switch (byte) {
            'i' => {
                const max_len = comptime std.fmt.comptimePrint("{}", .{std.math.minInt(i64)}).len;
                var s = reader.readUntilDelimiterAlloc(a, 'e', max_len) catch return error.Malformatted;
                defer a.free(s);
                const i = std.fmt.parseInt(i64, s, 10) catch return error.Malformatted;
                return .{ .int = i };
            },
            'l' => {
                var list = std.ArrayList(BValue).init(a);
                errdefer {
                    for (list.items) |*i| {
                        i.deinit(a);
                    }
                    list.deinit();
                }
                while (true) {
                    const b2 = try reader.readByte();
                    if (b2 == 'e') break;
                    try peekStream.putBackByte(b2);
                    var val = try bdecodeInner(a, peekStream, depth + 1);
                    errdefer val.deinit(a);
                    try list.append(val);
                }
                return .{ .list = list };
            },
            'd' => {
                var dict = std.StringArrayHashMap(BValue).init(a);
                errdefer {
                    var it = dict.iterator();
                    while (it.next()) |entry| {
                        a.free(entry.key_ptr.*);
                        entry.value_ptr.*.deinit(a);
                    }
                    dict.deinit();
                }
                lp: while (true) {
                    const b2 = try reader.readByte();
                    if (b2 == 'e') break :lp;
                    try peekStream.putBackByte(b2);
                    var key = try readString(a, peekStream);
                    errdefer a.free(key);
                    var val = try bdecode(a, reader);
                    errdefer val.deinit(a);
                    try dict.put(key, val);
                }
                return .{ .dict = dict };
            },
            else => return error.Malformatted, // TODO diagnostics
        }
    }
}

// Result is owned by the caller and must be freed
fn readString(a: std.mem.Allocator, peekStream: *PeekStream) ![]const u8 {
    var reader = peekStream.reader();
    const max_len = comptime std.fmt.comptimePrint("{}", .{std.math.maxInt(usize)}).len;
    const str_len_s = reader.readUntilDelimiterAlloc(a, ':', max_len) catch {
        return error.Malformatted;
    };
    defer a.free(str_len_s);
    var strlen = std.fmt.parseInt(usize, str_len_s, 10) catch return error.Malformatted;
    var string = try a.alloc(u8, strlen);
    errdefer a.free(string);
    reader.readNoEof(string) catch return error.Malformatted;
    return string;
}

test "bdecode empty" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.EndOfStream, bdecodeBuf(a, ""));
}

test "bdecode too short" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "1"));
}

test "bdecode plain number" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "12"));
}

test "bdecode garbage" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "xz1234"));
}

test "bdecode number" {
    var a = std.testing.allocator;
    var bval = try bdecodeBuf(a, "i123e");
    defer bval.deinit(a);
    try std.testing.expectEqualDeep(BValue{ .int = 123 }, bval);
}

test "bdecode number negative" {
    var a = std.testing.allocator;
    var bval = try bdecodeBuf(a, "i-123e");
    defer bval.deinit(a);
    try std.testing.expectEqualDeep(BValue{ .int = -123 }, bval);
}

test "bdecode number empty" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "ie"));
}

test "bdecode number just sign" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "i-e"));
}

test "bdecode number no end" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "i123123671283"));
}

test "bdecode number out of range" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "i9223372036854775808e"));
}

test "bdecode string" {
    var a = std.testing.allocator;
    var bval = try bdecodeBuf(a, "5:hello");
    defer bval.deinit(a);
    try std.testing.expectEqualDeep(BValue{ .string = "hello" }, bval);
}
test "bdecode string too short" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "5:hell"));
}

test "bdecode list" {
    var a = std.testing.allocator;
    var bval = try bdecodeBuf(a, "l5:hello5:worlde");
    defer bval.deinit(a);
    try std.testing.expectEqual(@as(usize, 2), bval.list.items.len);
    try std.testing.expectEqualStrings("hello", bval.list.items[0].string);
    try std.testing.expectEqualStrings("world", bval.list.items[1].string);
}

test "invalid list" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.EndOfStream, bdecodeBuf(a, "l5:hello5:world")); // missing end
}

test "dict" {
    var a = std.testing.allocator;
    var bval = try bdecodeBuf(a, "d5:hello5:worlde");
    defer bval.deinit(a);
    var v = bval.dict.getPtr("hello") orelse return error.TestExpectedNotNull;
    try std.testing.expectEqualStrings("world", v.string);
}

test "invalid dict no value" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "d5:hello5:world2:hie"));
}

test "invalid dict wrong key type" {
    var a = std.testing.allocator;
    try std.testing.expectError(error.Malformatted, bdecodeBuf(a, "di32e5:helloe"));
}

test "nested structure" {
    var a = std.testing.allocator;
    var bval = try bdecodeBuf(a, "d5:hello5:world2:hili123ei456el4:nesteee");
    defer bval.deinit(a);
    var v = bval.dict.getPtr("hello") orelse return error.TestExpectedNotNull;
    try std.testing.expectEqualStrings("world", v.string);
    var v2 = bval.dict.getPtr("hi") orelse return error.TestExpectedNotNull;
    try std.testing.expectEqualDeep(v2.*.list.items[0], BValue{ .int = 123 });
    try std.testing.expectEqualDeep(v2.*.list.items[1], BValue{ .int = 456 });
    try std.testing.expectEqualStrings("nest", v2.*.list.items[2].list.items[0].string);
}