// SPDX-FileCopyrightText: © 2026 Jeffrey C. Ollie <jeff@ocjtech.us>
// SPDX-License-Identifier: MIT

//! Shared SMTP protocol primitives (RFC 5321): line reading, reply parsing,
//! command parsing, and message data dot-stuffing. Used by both the client
//! and server layers, and usable directly for custom protocol handling.

const std = @import("std");
const Io = std.Io;

pub const crlf = "\r\n";

pub const ReadLineError = error{
    ReadFailed,
    EndOfStream,
    /// The line did not fit in the reader's buffer.
    LineTooLong,
};

/// Reads one CRLF- (or bare LF-) terminated line, returning it without the
/// line ending. The returned slice points into the reader's buffer and is
/// invalidated by the next read.
pub fn readLine(reader: *Io.Reader) ReadLineError![]u8 {
    const line = reader.takeSentinel('\n') catch |err| switch (err) {
        error.StreamTooLong => return error.LineTooLong,
        error.ReadFailed, error.EndOfStream => |e| return e,
    };
    if (line.len > 0 and line[line.len - 1] == '\r') return line[0 .. line.len - 1];
    return line;
}

/// A server reply: a 3-digit code and one or more lines of text.
pub const Reply = struct {
    code: u16,
    /// Text of all reply lines joined with '\n', with codes and separators
    /// stripped. Points into the buffer passed to `read`.
    text: []const u8,

    pub const ReadError = ReadLineError || error{
        InvalidReply,
        /// The reply text did not fit in the provided buffer.
        ReplyTooLong,
    };

    /// Reads one (possibly multiline) reply. The text is copied into `buffer`
    /// and the returned reply's `text` field points into it.
    pub fn read(reader: *Io.Reader, buffer: []u8) ReadError!Reply {
        var text: Io.Writer = .fixed(buffer);
        var code: ?u16 = null;
        var first = true;
        while (true) {
            const line = try readLine(reader);
            if (line.len < 3) return error.InvalidReply;
            const line_code = std.fmt.parseInt(u16, line[0..3], 10) catch
                return error.InvalidReply;
            if (line_code < 100 or line_code > 599) return error.InvalidReply;
            if (code) |prev| {
                // All lines of a multiline reply must carry the same code.
                if (prev != line_code) return error.InvalidReply;
            } else {
                code = line_code;
            }
            var last = true;
            var line_text: []const u8 = "";
            if (line.len > 3) {
                switch (line[3]) {
                    ' ' => {},
                    '-' => last = false,
                    else => return error.InvalidReply,
                }
                line_text = line[4..];
            }
            if (!first) text.writeByte('\n') catch return error.ReplyTooLong;
            text.writeAll(line_text) catch return error.ReplyTooLong;
            first = false;
            if (last) break;
        }
        return .{ .code = code.?, .text = text.buffered() };
    }

    /// Iterates over the individual text lines of the reply.
    pub fn lines(r: *const Reply) std.mem.SplitIterator(u8, .scalar) {
        return std.mem.splitScalar(u8, r.text, '\n');
    }

    // Reply classes per RFC 5321 §4.2.1.
    pub fn isPositiveCompletion(r: Reply) bool {
        return r.code >= 200 and r.code < 300;
    }
    pub fn isPositiveIntermediate(r: Reply) bool {
        return r.code >= 300 and r.code < 400;
    }
    pub fn isTransientFailure(r: Reply) bool {
        return r.code >= 400 and r.code < 500;
    }
    pub fn isPermanentFailure(r: Reply) bool {
        return r.code >= 500 and r.code < 600;
    }
};

/// A parsed client command, as seen by a server.
pub const Command = union(enum) {
    helo: []const u8,
    ehlo: []const u8,
    /// MAIL FROM. An empty path is the null reverse-path (`MAIL FROM:<>`).
    mail: PathArgs,
    /// RCPT TO.
    rcpt: PathArgs,
    data,
    rset,
    noop,
    quit,
    vrfy: []const u8,
    help,
    starttls,
    /// AUTH (RFC 4954).
    auth: AuthArgs,
    /// Unrecognized command verb; the payload is the full line.
    unknown: []const u8,

    pub const AuthArgs = struct {
        mechanism: []const u8,
        /// Raw base64 initial response, if the client sent one ("=" denotes
        /// an empty initial response).
        initial: []const u8 = "",
    };

    pub const PathArgs = struct {
        /// The mailbox, with angle brackets and any obsolete source route
        /// stripped.
        path: []const u8,
        /// Raw ESMTP parameters that followed the path, e.g. "SIZE=1024".
        params: []const u8 = "",
    };

    pub const ParseError = error{Syntax};

    /// Parses one command line (without its line ending). Returned slices
    /// point into `line`.
    pub fn parse(line: []const u8) ParseError!Command {
        const trimmed = std.mem.trim(u8, line, " \t");
        const verb_end = std.mem.indexOfAny(u8, trimmed, " \t") orelse trimmed.len;
        const verb = trimmed[0..verb_end];
        const rest = std.mem.trimStart(u8, trimmed[verb_end..], " \t");

        if (ieql(verb, "HELO")) {
            if (rest.len == 0) return error.Syntax;
            return .{ .helo = rest };
        }
        if (ieql(verb, "EHLO")) {
            if (rest.len == 0) return error.Syntax;
            return .{ .ehlo = rest };
        }
        if (ieql(verb, "MAIL")) return .{ .mail = try parsePathArgs(rest, "FROM:") };
        if (ieql(verb, "RCPT")) return .{ .rcpt = try parsePathArgs(rest, "TO:") };
        if (ieql(verb, "DATA")) return .data;
        if (ieql(verb, "RSET")) return .rset;
        if (ieql(verb, "NOOP")) return .noop;
        if (ieql(verb, "QUIT")) return .quit;
        if (ieql(verb, "VRFY")) return .{ .vrfy = rest };
        if (ieql(verb, "HELP")) return .help;
        if (ieql(verb, "STARTTLS")) return .starttls;
        if (ieql(verb, "AUTH")) {
            const mech_end = std.mem.indexOfAny(u8, rest, " \t") orelse rest.len;
            if (mech_end == 0) return error.Syntax;
            return .{ .auth = .{
                .mechanism = rest[0..mech_end],
                .initial = std.mem.trimStart(u8, rest[mech_end..], " \t"),
            } };
        }
        return .{ .unknown = line };
    }

    fn parsePathArgs(rest: []const u8, comptime keyword: []const u8) ParseError!PathArgs {
        if (rest.len < keyword.len or !ieql(rest[0..keyword.len], keyword))
            return error.Syntax;
        const after = std.mem.trimStart(u8, rest[keyword.len..], " \t");
        if (after.len == 0 or after[0] != '<') {
            // Lenient: accept a bare address ending at whitespace.
            const end = std.mem.indexOfAny(u8, after, " \t") orelse after.len;
            if (end == 0) return error.Syntax;
            return .{
                .path = after[0..end],
                .params = std.mem.trimStart(u8, after[end..], " \t"),
            };
        }
        const close = std.mem.indexOfScalar(u8, after, '>') orelse return error.Syntax;
        var path = after[1..close];
        // Strip an obsolete source route: <@relay1,@relay2:user@host>.
        if (path.len > 0 and path[0] == '@') {
            const colon = std.mem.indexOfScalar(u8, path, ':') orelse return error.Syntax;
            path = path[colon + 1 ..];
        }
        return .{
            .path = path,
            .params = std.mem.trimStart(u8, after[close + 1 ..], " \t"),
        };
    }

    fn ieql(a: []const u8, b: []const u8) bool {
        return std.ascii.eqlIgnoreCase(a, b);
    }
};

/// Writes `data` as SMTP message content: line endings are normalized to CRLF
/// and lines beginning with '.' are dot-stuffed (RFC 5321 §4.5.2). Does not
/// write the terminating ".\r\n".
pub fn writeStuffed(writer: *Io.Writer, data: []const u8) Io.Writer.Error!void {
    var rest = data;
    while (rest.len > 0) {
        var line: []const u8 = undefined;
        if (std.mem.indexOfScalar(u8, rest, '\n')) |i| {
            line = rest[0..i];
            rest = rest[i + 1 ..];
        } else {
            line = rest;
            rest = rest[rest.len..];
        }
        if (line.len > 0 and line[line.len - 1] == '\r') line = line[0 .. line.len - 1];
        if (line.len > 0 and line[0] == '.') try writer.writeByte('.');
        try writer.writeAll(line);
        try writer.writeAll(crlf);
    }
}

test "readLine strips CRLF and LF" {
    var reader: Io.Reader = .fixed("first\r\nsecond\nthird\r\n");
    try std.testing.expectEqualStrings("first", try readLine(&reader));
    try std.testing.expectEqualStrings("second", try readLine(&reader));
    try std.testing.expectEqualStrings("third", try readLine(&reader));
    try std.testing.expectError(error.EndOfStream, readLine(&reader));
}

test "Reply.read single line" {
    var reader: Io.Reader = .fixed("250 2.0.0 Ok\r\n");
    var buf: [128]u8 = undefined;
    const reply = try Reply.read(&reader, &buf);
    try std.testing.expectEqual(@as(u16, 250), reply.code);
    try std.testing.expectEqualStrings("2.0.0 Ok", reply.text);
    try std.testing.expect(reply.isPositiveCompletion());
}

test "Reply.read multiline" {
    var reader: Io.Reader = .fixed("250-mx.example.com\r\n250-PIPELINING\r\n250 SIZE 1000\r\n");
    var buf: [128]u8 = undefined;
    const reply = try Reply.read(&reader, &buf);
    try std.testing.expectEqual(@as(u16, 250), reply.code);
    try std.testing.expectEqualStrings("mx.example.com\nPIPELINING\nSIZE 1000", reply.text);
    var it = reply.lines();
    try std.testing.expectEqualStrings("mx.example.com", it.next().?);
    try std.testing.expectEqualStrings("PIPELINING", it.next().?);
    try std.testing.expectEqualStrings("SIZE 1000", it.next().?);
    try std.testing.expectEqual(@as(?[]const u8, null), it.next());
}

test "Reply.read rejects malformed replies" {
    var buf: [128]u8 = undefined;
    {
        var reader: Io.Reader = .fixed("2x0 hello\r\n");
        try std.testing.expectError(error.InvalidReply, Reply.read(&reader, &buf));
    }
    {
        var reader: Io.Reader = .fixed("250-one\r\n251 two\r\n");
        try std.testing.expectError(error.InvalidReply, Reply.read(&reader, &buf));
    }
    {
        var reader: Io.Reader = .fixed("42\r\n");
        try std.testing.expectError(error.InvalidReply, Reply.read(&reader, &buf));
    }
}

test "Command.parse" {
    {
        const cmd = try Command.parse("EHLO client.example.com");
        try std.testing.expectEqualStrings("client.example.com", cmd.ehlo);
    }
    {
        const cmd = try Command.parse("mail from:<alice@example.com> SIZE=1024");
        try std.testing.expectEqualStrings("alice@example.com", cmd.mail.path);
        try std.testing.expectEqualStrings("SIZE=1024", cmd.mail.params);
    }
    {
        // Null reverse-path and a space after the colon.
        const cmd = try Command.parse("MAIL FROM: <>");
        try std.testing.expectEqualStrings("", cmd.mail.path);
    }
    {
        // Obsolete source route is stripped.
        const cmd = try Command.parse("RCPT TO:<@relay.example:bob@example.net>");
        try std.testing.expectEqualStrings("bob@example.net", cmd.rcpt.path);
    }
    {
        const cmd = try Command.parse("QUIT");
        try std.testing.expectEqual(Command.quit, cmd);
    }
    {
        const cmd = try Command.parse("AUTH PLAIN AHVzZXIAcGFzcw==");
        try std.testing.expectEqualStrings("PLAIN", cmd.auth.mechanism);
        try std.testing.expectEqualStrings("AHVzZXIAcGFzcw==", cmd.auth.initial);
    }
    {
        const cmd = try Command.parse("auth login");
        try std.testing.expectEqualStrings("login", cmd.auth.mechanism);
        try std.testing.expectEqualStrings("", cmd.auth.initial);
    }
    {
        const cmd = try Command.parse("MADE UP");
        try std.testing.expectEqualStrings("MADE UP", cmd.unknown);
    }
    try std.testing.expectError(error.Syntax, Command.parse("AUTH"));
    try std.testing.expectError(error.Syntax, Command.parse("MAIL TO:<a@b>"));
    try std.testing.expectError(error.Syntax, Command.parse("RCPT TO:"));
    try std.testing.expectError(error.Syntax, Command.parse("HELO"));
}

test "writeStuffed" {
    var buf: [256]u8 = undefined;
    {
        var w: Io.Writer = .fixed(&buf);
        try writeStuffed(&w, "line one\r\n.starts with dot\r\n");
        try std.testing.expectEqualStrings("line one\r\n..starts with dot\r\n", w.buffered());
    }
    {
        // LF-only input is normalized, missing final newline is added.
        var w: Io.Writer = .fixed(&buf);
        try writeStuffed(&w, "a\nb");
        try std.testing.expectEqualStrings("a\r\nb\r\n", w.buffered());
    }
    {
        // A lone "." line must not become a terminator.
        var w: Io.Writer = .fixed(&buf);
        try writeStuffed(&w, ".\n");
        try std.testing.expectEqualStrings("..\r\n", w.buffered());
    }
    {
        var w: Io.Writer = .fixed(&buf);
        try writeStuffed(&w, "");
        try std.testing.expectEqualStrings("", w.buffered());
    }
}