#!/usr/bin/perl
# SPDX-FileCopyrightText: Amolith <amolith@secluded.site>
#
# SPDX-License-Identifier: GPL-3.0-or-later

use strict;
use warnings;

my $WIDTH = 72;

my $mode = shift @ARGV;
unless (defined $mode && ($mode eq 'subject' || $mode eq 'message')) {
    print STDERR "Usage: format subject \"text\" | format message < file\n";
    exit 1;
}

if ($mode eq 'subject') {
    my $subject = shift @ARGV;
    unless (defined $subject) {
        print STDERR "subject mode requires a subject argument\n";
        exit 1;
    }
    if (length($subject) > 50) {
        print STDERR "Subject exceeds 50 characters (" . length($subject) . ")\n";
        exit 1;
    }
    print $subject;
    exit 0;
}

# Message mode: read full message from stdin, format it, emit to stdout.
my @all_lines = <STDIN>;
chomp @all_lines;

unless (@all_lines) {
    print STDERR "Empty message\n";
    exit 1;
}

# First line is the subject.
my $subject = $all_lines[0];
if (length($subject) > 50) {
    print STDERR "Subject exceeds 50 characters (" . length($subject) . ")\n";
    exit 1;
}

# Subject only — no body or trailers.
if (@all_lines == 1) {
    print "$subject\n";
    exit 0;
}

# Skip the blank line after the subject.
my $start = 1;
$start++ if $start < @all_lines && $all_lines[$start] =~ /^\s*$/;

# Nothing after the subject (just trailing blanks).
if ($start >= @all_lines) {
    print "$subject\n";
    exit 0;
}

my @rest = @all_lines[$start .. $#all_lines];

# Detect trailer block by scanning backward. Trailers are Key: value lines at
# the end of the message (after skipping trailing blank lines). The key must
# start with a letter and contain only letters, digits, and hyphens.
my $scan = $#rest;

# Skip trailing blank lines.
$scan-- while $scan >= 0 && $rest[$scan] =~ /^\s*$/;

my $trailer_end = $scan;  # last non-blank line

# Collect trailer lines.
while ($scan >= 0 && $rest[$scan] =~ /^[A-Za-z][A-Za-z0-9-]*: ./) {
    $scan--;
}

my $trailer_start = $scan + 1;
my @trailers;
if ($trailer_start <= $trailer_end) {
    @trailers = @rest[$trailer_start .. $trailer_end];
}

# Body is everything before the trailer block, with trailing blanks trimmed.
my @body_lines;
if ($trailer_start > 0) {
    my $body_end = $trailer_start - 1;
    # Trim trailing blank lines from the body.
    $body_end-- while $body_end >= 0 && $rest[$body_end] =~ /^\s*$/;
    @body_lines = @rest[0 .. $body_end] if $body_end >= 0;
}

# Reflow the body.
my $formatted_body = reflow_body(@body_lines);

# Assemble output.
print "$subject\n";
if ($formatted_body ne '') {
    print "\n$formatted_body\n";
}
if (@trailers) {
    print "\n", join("\n", @trailers), "\n";
}

exit 0;

# --- Subroutines ---

sub reflow_body {
    my @lines = @_;
    return '' unless @lines;

    my @result;
    my @plain_buffer;

    my $flush_plain = sub {
        if (@plain_buffer) {
            my $joined = join(' ', @plain_buffer);
            push @result, word_wrap($joined, $WIDTH);
            @plain_buffer = ();
        }
    };

    for my $line (@lines) {
        # Code blocks: 4+ leading spaces or tab — preserve as-is.
        if ($line =~ /^(?:    |\t)/) {
            $flush_plain->();
            push @result, $line;
            next;
        }

        (my $trimmed = $line) =~ s/^\s+|\s+$//g;

        # Blank lines.
        if ($trimmed eq '') {
            $flush_plain->();
            push @result, '';
            next;
        }

        # Bullet lists (- or *).
        if ($trimmed =~ /^([*-] )(.*)/) {
            $flush_plain->();
            my ($marker, $content) = ($1, $2);
            push @result, wrap_hanging($marker, '  ', $content, $WIDTH);
            next;
        }

        # Numbered lists (1. , 2. , 10. , etc.).
        if ($trimmed =~ /^(\d+\.\s)(.*)/) {
            $flush_plain->();
            my $marker = $1;
            my $content = defined $2 ? $2 : '';
            my $indent = ' ' x length($marker);
            push @result, wrap_hanging($marker, $indent, $content, $WIDTH);
            next;
        }

        # Plain text: buffer for paragraph reflow.
        push @plain_buffer, $trimmed;
    }

    $flush_plain->();
    return join("\n", @result);
}

sub word_wrap {
    my ($text, $width) = @_;
    my @words = split /\s+/, $text;
    return '' unless @words;

    my @lines;
    my $current = '';
    my $current_width = 0;

    for my $word (@words) {
        my $wlen = length($word);
        if ($current eq '') {
            $current = $word;
            $current_width = $wlen;
        } elsif ($current_width + 1 + $wlen <= $width) {
            $current .= " $word";
            $current_width += 1 + $wlen;
        } else {
            push @lines, $current;
            $current = $word;
            $current_width = $wlen;
        }
    }
    push @lines, $current if $current ne '';
    return join("\n", @lines);
}

sub wrap_hanging {
    my ($first_prefix, $cont_prefix, $text, $width) = @_;
    my $first_width = $width - length($first_prefix);
    my $cont_width  = $width - length($cont_prefix);

    my @words = split /\s+/, $text;
    return $first_prefix unless @words;

    my @lines;
    my $current = '';
    my $current_width = 0;
    my $is_first = 1;

    for my $word (@words) {
        my $wlen = length($word);
        my $max = $is_first ? $first_width : $cont_width;

        if ($current eq '') {
            $current = $word;
            $current_width = $wlen;
        } elsif ($current_width + 1 + $wlen <= $max) {
            $current .= " $word";
            $current_width += 1 + $wlen;
        } else {
            my $prefix = $is_first ? $first_prefix : $cont_prefix;
            push @lines, $prefix . $current;
            $is_first = 0;
            $current = $word;
            $current_width = $wlen;
        }
    }

    if ($current ne '') {
        my $prefix = $is_first ? $first_prefix : $cont_prefix;
        push @lines, $prefix . $current;
    }

    return join("\n", @lines);
}