const std = @import("std");

/// Parsed SSE event data — the content after "data: " (prefix removed).
/// For multi-line data events, lines are joined with newlines per the SSE spec.
///
/// Borrowed from the parser's internal buffer. Call `SSEParser.freeEvents()`
/// to free the events list when done (event data itself is borrowed and
/// does not need individual freeing).
const Event = []const u8;

/// Incremental SSE line parser.
///
/// The HTTP client delivers arbitrary-sized read buffers; this module
/// reassembles them into complete `data: ...\n\n` events.
///
/// Event data is borrowed from the internal buffer. Call `freeEvents()` to
/// free the events list returned by `feed`; the event data itself is borrowed
/// from the parser's buffer and does not need individual freeing.
pub const SSEParser = struct {
    buf: std.ArrayList(u8) = .empty,
    allocator: std.mem.Allocator,

    pub fn init(allocator: std.mem.Allocator) SSEParser {
        return .{
            .allocator = allocator,
        };
    }

    /// Feed a chunk of raw bytes. Returns a list of complete SSE events
    /// found in the buffer. Returns an empty list if no complete events
    /// are available yet.
    ///
    /// Event data is borrowed from the internal buffer. Call `freeEvents()`
    /// on the returned list when done.
    pub fn feed(self: *SSEParser, chunk: []const u8) ![]Event {
        try self.buf.appendSlice(self.allocator, chunk);

        var events = std.ArrayList(Event).init(self.allocator);
        errdefer events.deinit();

        var write_pos: usize = 0;
        var scan_pos: usize = 0;

        while (std.mem.indexOf(u8, self.buf.items[scan_pos..], "\n\n")) |rel_delim| {
            const delim = scan_pos + rel_delim;
            const event_bytes = self.buf.items[scan_pos..delim];

            // Compact data lines in-place. Compacted data is always shorter
            // than the original event bytes (we strip at least "data: " per
            // line), so write_pos <= scan_pos and we never overwrite data
            // we haven't yet read.
            const data_start = write_pos;
            var line_start: usize = 0;
            var has_data = false;

            while (line_start < event_bytes.len) {
                const newline = std.mem.indexOfScalarPos(u8, event_bytes, line_start, '\n') orelse event_bytes.len;
                const line = event_bytes[line_start..newline];

                if (std.mem.startsWith(u8, line, "data: ")) {
                    if (has_data) {
                        self.buf.items[write_pos] = '\n';
                        write_pos += 1;
                    }
                    const content = line["data: ".len..];
                    std.mem.copyForwards(u8, self.buf.items[write_pos .. write_pos + content.len], content);
                    write_pos += content.len;
                    has_data = true;
                }
                // Ignore other SSE fields (event:, id:, retry:) and comments (starting with :)

                line_start = newline + 1;
            }

            if (has_data) {
                try events.append(self.buf.items[data_start..write_pos]);
            }

            scan_pos = delim + 2;
        }

        // Shift remaining (incomplete) bytes to the front, retaining
        // the backing allocation for reuse.
        const remaining_len = self.buf.items.len - scan_pos;
        if (remaining_len > 0 and scan_pos > 0) {
            std.mem.copyForwards(u8, self.buf.items[0..remaining_len], self.buf.items[scan_pos..]);
        }
        self.buf.shrinkRetainingCapacity(remaining_len);

        return events.toOwnedSlice();
    }

    /// Free a list of events returned by `feed`.
    /// Event data is borrowed from the parser's buffer and does not need
    /// to be freed individually; only the events array itself is freed.
    pub fn freeEvents(self: *SSEParser, events: []Event) void {
        self.allocator.free(events);
    }

    pub fn deinit(self: *SSEParser) void {
        self.buf.deinit(self.allocator);
    }
};

test "SSEParser - single complete event" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    const events = try parser.feed("data: hello\n\n");
    defer parser.freeEvents(events);

    try std.testing.expectEqual(@as(usize, 1), events.len);
    try std.testing.expectEqualStrings("hello", events[0]);
}

test "SSEParser - partial then complete" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    // Feed partial — no complete event yet
    const events1 = try parser.feed("data: hel");
    defer parser.freeEvents(events1);
    try std.testing.expectEqual(@as(usize, 0), events1.len);

    // Feed remainder — completes the event
    const events2 = try parser.feed("lo\n\n");
    defer parser.freeEvents(events2);
    try std.testing.expectEqual(@as(usize, 1), events2.len);
    try std.testing.expectEqualStrings("hello", events2[0]);
}

test "SSEParser - multiple events in single chunk" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    const events = try parser.feed("data: one\n\ndata: two\n\n");
    defer parser.freeEvents(events);

    try std.testing.expectEqual(@as(usize, 2), events.len);
    try std.testing.expectEqualStrings("one", events[0]);
    try std.testing.expectEqualStrings("two", events[1]);
}

test "SSEParser - data DONE signal" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    const events = try parser.feed("data: [DONE]\n\n");
    defer parser.freeEvents(events);

    try std.testing.expectEqual(@as(usize, 1), events.len);
    try std.testing.expectEqualStrings("[DONE]", events[0]);
}

test "SSEParser - empty event (keep-alive)" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    // A blank line (just \n\n) is a keep-alive comment with no data
    const events = try parser.feed("\n\n");
    defer parser.freeEvents(events);
    try std.testing.expectEqual(@as(usize, 0), events.len);
}

test "SSEParser - ignores non-data fields" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    const events = try parser.feed("event: message\ndata: payload\nid: 42\n\n");
    defer parser.freeEvents(events);

    try std.testing.expectEqual(@as(usize, 1), events.len);
    try std.testing.expectEqualStrings("payload", events[0]);
}

test "SSEParser - multi-line data joined with newline" {
    const allocator = std.testing.allocator;

    var parser = SSEParser.init(allocator);
    defer parser.deinit();

    const events = try parser.feed("data: line1\ndata: line2\n\n");
    defer parser.freeEvents(events);

    try std.testing.expectEqual(@as(usize, 1), events.len);
    try std.testing.expectEqualStrings("line1\nline2", events[0]);
}