//! Versioned, comment-only metadata immediately attached to a WireGuard peer.
//!
//! The alias is encoded as canonical unpadded base64url of its UTF-8 bytes. This
//! keeps arbitrary valid aliases on one comment line without introducing config
//! syntax, comment termination, or line injection.

use base64::Engine;
use base64::engine::general_purpose::URL_SAFE_NO_PAD;

use crate::domain::validate_alias;

use super::SourceSpan;

const BEGIN: &str = "# wg-admin:begin";
const END: &str = "# wg-admin:end";
const VERSION_PREFIX: &str = "# wg-admin:version=";
const ALIAS_PREFIX: &str = "# wg-admin:alias=";
const CREATED_PREFIX: &str = "# wg-admin:created-utc=";

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ManagedMetadata {
    pub version: u32,
    pub alias: String,
    pub created_utc: Option<String>,
}

/// A safe half-open source span for a validated, uniquely attached block.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum MetadataAttachment {
    Absent,
    Valid {
        metadata: ManagedMetadata,
        block: SourceSpan,
    },
    Invalid(MetadataIssue),
}

/// Redacted metadata problems. Variants identify a line, never include its text.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum MetadataIssue {
    MalformedBlock { line: usize },
    UnsupportedVersion { line: usize },
    InvalidAlias { line: usize },
    InvalidTimestamp { line: usize },
    UnknownField { line: usize },
    MissingField { line: usize },
    DuplicateBlocks { line: usize, count: usize },
    Misattached { line: usize },
}

#[derive(Clone, Debug)]
struct Line<'a> {
    content: &'a str,
    start: usize,
    end: usize,
    number: usize,
}

#[derive(Clone, Debug)]
struct Candidate {
    start_line: usize,
    end_line: usize,
    span: SourceSpan,
    issue: Option<MetadataIssue>,
}

/// Inspect all reserved marker pairs and attach blocks only when their final byte
/// is the peer header's first byte. Administrator comments outside this namespace
/// are ignored completely.
pub(super) fn inspect_document(
    source: &str,
    peer_headers: &[usize],
) -> (Vec<MetadataAttachment>, Vec<MetadataIssue>) {
    let lines = lines(source);
    let mut candidates = Vec::new();
    let mut issues = Vec::new();
    let mut index = 0;
    while index < lines.len() {
        if marker(lines[index].content) != Some(Marker::Begin) {
            if marker(lines[index].content) == Some(Marker::End) {
                issues.push(MetadataIssue::MalformedBlock {
                    line: lines[index].number,
                });
            }
            index += 1;
            continue;
        }
        let start = index;
        let mut end = None;
        let mut malformed = false;
        let mut cursor = index + 1;
        while cursor < lines.len() {
            match marker(lines[cursor].content) {
                Some(Marker::Begin) => {
                    malformed = true;
                    break;
                }
                Some(Marker::End) => {
                    end = Some(cursor);
                    break;
                }
                None => {}
            }
            cursor += 1;
        }
        let attached_peer = peer_headers.iter().find(|header| {
            **header > lines[start].start
                && (end.is_none()
                    || **header <= lines[cursor.min(lines.len().saturating_sub(1))].start)
        });
        let block_end_line = end.unwrap_or_else(|| {
            if malformed {
                cursor
            } else if let Some(header) = attached_peer {
                lines
                    .partition_point(|line| line.start < *header)
                    .saturating_sub(1)
            } else {
                lines.len().saturating_sub(1)
            }
        });
        let span_end = if let Some(end_line) = end {
            lines[end_line].end
        } else if malformed {
            lines[block_end_line].start
        } else if let Some(header) = attached_peer {
            *header
        } else {
            source.len()
        };
        let Some(span) = SourceSpan::from_offsets(lines[start].start, span_end) else {
            index += 1;
            continue;
        };
        let issue = if malformed || end.is_none() {
            Some(MetadataIssue::MalformedBlock {
                line: lines[start].number,
            })
        } else {
            parse_block(&lines[start..=block_end_line]).err()
        };
        if let Some(issue) = &issue {
            issues.push(issue.clone());
        }
        candidates.push(Candidate {
            start_line: start,
            end_line: block_end_line,
            span,
            issue,
        });
        if let Some(end) = end {
            index = end + 1;
        } else if malformed {
            index = cursor;
        } else {
            break;
        }
    }

    let mut attachments = Vec::with_capacity(peer_headers.len());
    for header in peer_headers {
        let ending_here: Vec<_> = candidates
            .iter()
            .enumerate()
            .filter(|(_, candidate)| candidate.span.end() == *header)
            .map(|(candidate, _)| candidate)
            .collect();
        if ending_here.is_empty() {
            attachments.push(MetadataAttachment::Absent);
            continue;
        }
        let Some(last) = ending_here.last().copied() else {
            attachments.push(MetadataAttachment::Absent);
            continue;
        };
        let mut contiguous = 1;
        let mut previous = last;
        while previous > 0
            && candidates[previous - 1].span.end() == candidates[previous].span.start()
        {
            contiguous += 1;
            previous -= 1;
        }
        let candidate = &candidates[last];
        if contiguous > 1 {
            let issue = MetadataIssue::DuplicateBlocks {
                line: lines[candidate.start_line].number,
                count: contiguous,
            };
            issues.push(issue.clone());
            attachments.push(MetadataAttachment::Invalid(issue));
        } else if let Some(issue) = &candidate.issue {
            attachments.push(MetadataAttachment::Invalid(issue.clone()));
        } else {
            let parsed = parse_block(&lines[candidate.start_line..=candidate.end_line]);
            match parsed {
                Ok(metadata) => attachments.push(MetadataAttachment::Valid {
                    metadata,
                    block: candidate.span,
                }),
                Err(issue) => {
                    attachments.push(MetadataAttachment::Invalid(issue));
                }
            }
        }
    }

    for candidate in &candidates {
        if !peer_headers.contains(&candidate.span.end()) {
            let issue = MetadataIssue::Misattached {
                line: lines[candidate.start_line].number,
            };
            issues.push(issue);
        }
    }
    (attachments, issues)
}

/// Render the canonical LF-terminated v1 block. The caller may adapt line endings
/// when inserting it into a document; this function performs no source mutation.
pub fn render_block(alias: &str, created_utc: Option<&str>) -> Result<String, MetadataError> {
    validate_alias(alias).map_err(|_| MetadataError::InvalidAlias)?;
    if let Some(timestamp) = created_utc
        && !valid_utc_timestamp(timestamp)
    {
        return Err(MetadataError::InvalidTimestamp);
    }
    let encoded = URL_SAFE_NO_PAD.encode(alias.as_bytes());
    let mut block = format!("{BEGIN}\n{VERSION_PREFIX}1\n{ALIAS_PREFIX}{encoded}\n");
    if let Some(timestamp) = created_utc {
        block.push_str(CREATED_PREFIX);
        block.push_str(timestamp);
        block.push('\n');
    }
    block.push_str(END);
    block.push('\n');
    Ok(block)
}

#[derive(Clone, Copy, Debug, Eq, PartialEq, thiserror::Error)]
pub enum MetadataError {
    #[error("client alias is invalid")]
    InvalidAlias,
    #[error("creation timestamp must be a valid UTC timestamp")]
    InvalidTimestamp,
}

fn parse_block(lines: &[Line<'_>]) -> Result<ManagedMetadata, MetadataIssue> {
    let first_line = lines.first().map_or(1, |line| line.number);
    if lines.len() < 4 || lines[0].content != BEGIN || lines[lines.len() - 1].content != END {
        return Err(MetadataIssue::MalformedBlock { line: first_line });
    }
    let version_line = &lines[1];
    let Some(version_text) = version_line.content.strip_prefix(VERSION_PREFIX) else {
        return Err(MetadataIssue::MissingField { line: first_line });
    };
    if version_text.is_empty() || !version_text.bytes().all(|byte| byte.is_ascii_digit()) {
        return Err(MetadataIssue::MalformedBlock {
            line: version_line.number,
        });
    }
    let version = version_text
        .parse::<u32>()
        .map_err(|_| MetadataIssue::MalformedBlock {
            line: version_line.number,
        })?;
    if version != 1 {
        return Err(MetadataIssue::UnsupportedVersion {
            line: version_line.number,
        });
    }
    let alias_line = &lines[2];
    let Some(encoded_alias) = alias_line.content.strip_prefix(ALIAS_PREFIX) else {
        return Err(MetadataIssue::MissingField {
            line: alias_line.number,
        });
    };
    let decoded =
        URL_SAFE_NO_PAD
            .decode(encoded_alias)
            .map_err(|_| MetadataIssue::InvalidAlias {
                line: alias_line.number,
            })?;
    if URL_SAFE_NO_PAD.encode(&decoded) != encoded_alias {
        return Err(MetadataIssue::InvalidAlias {
            line: alias_line.number,
        });
    }
    let alias = String::from_utf8(decoded).map_err(|_| MetadataIssue::InvalidAlias {
        line: alias_line.number,
    })?;
    validate_alias(&alias).map_err(|_| MetadataIssue::InvalidAlias {
        line: alias_line.number,
    })?;
    let created_lines = &lines[3..lines.len() - 1];
    let created_utc = match created_lines {
        [] => None,
        [line] => {
            let Some(value) = line.content.strip_prefix(CREATED_PREFIX) else {
                return Err(if line.content.starts_with("# wg-admin:") {
                    MetadataIssue::UnknownField { line: line.number }
                } else {
                    MetadataIssue::MalformedBlock { line: line.number }
                });
            };
            if !valid_utc_timestamp(value) {
                return Err(MetadataIssue::InvalidTimestamp { line: line.number });
            }
            Some(value.to_owned())
        }
        [line, ..] => {
            return Err(if line.content.starts_with("# wg-admin:") {
                MetadataIssue::UnknownField { line: line.number }
            } else {
                MetadataIssue::MalformedBlock { line: line.number }
            });
        }
    };
    Ok(ManagedMetadata {
        version,
        alias,
        created_utc,
    })
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum Marker {
    Begin,
    End,
}

fn marker(line: &str) -> Option<Marker> {
    if line == BEGIN
        || line.starts_with(&format!("{BEGIN} "))
        || line.starts_with(&format!("{BEGIN}\t"))
    {
        Some(Marker::Begin)
    } else if line == END
        || line.starts_with(&format!("{END} "))
        || line.starts_with(&format!("{END}\t"))
    {
        Some(Marker::End)
    } else {
        None
    }
}

fn lines(source: &str) -> Vec<Line<'_>> {
    let mut result = Vec::new();
    let mut offset = 0;
    for (index, raw) in source.split_inclusive('\n').enumerate() {
        let end = offset + raw.len();
        let content = raw.strip_suffix('\n').unwrap_or(raw);
        let content = content.strip_suffix('\r').unwrap_or(content);
        result.push(Line {
            content,
            start: offset,
            end,
            number: index + 1,
        });
        offset = end;
    }
    if source.is_empty() {
        return result;
    }
    result
}

fn valid_utc_timestamp(value: &str) -> bool {
    if value.len() != 20
        || !value.is_ascii()
        || value.as_bytes()[4] != b'-'
        || value.as_bytes()[7] != b'-'
        || value.as_bytes()[10] != b'T'
        || value.as_bytes()[13] != b':'
        || value.as_bytes()[16] != b':'
        || value.as_bytes()[19] != b'Z'
    {
        return false;
    }
    let digits = |range: std::ops::Range<usize>| value[range].parse::<u32>().ok();
    let (Some(year), Some(month), Some(day), Some(hour), Some(minute), Some(second)) = (
        digits(0..4),
        digits(5..7),
        digits(8..10),
        digits(11..13),
        digits(14..16),
        digits(17..19),
    ) else {
        return false;
    };
    if year == 0 || !(1..=12).contains(&month) || hour > 23 || minute > 59 || second > 59 {
        return false;
    }
    let leap = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
    let days = match month {
        2 if leap => 29,
        2 => 28,
        4 | 6 | 9 | 11 => 30,
        _ => 31,
    };
    (1..=days).contains(&day)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn wireguard_metadata_round_trips_aliases_and_optional_timestamp() {
        let alias = "# café = \"laptop\" <tag>";
        for timestamp in [None, Some("2024-02-29T23:59:59Z")] {
            let rendered = render_block(alias, timestamp).unwrap();
            let header = rendered.len();
            let (attachments, issues) = inspect_document(
                &format!(
                    "{rendered}[Peer]\nPublicKey = AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=\n"
                ),
                &[header],
            );
            assert!(issues.is_empty());
            let MetadataAttachment::Valid { metadata, block } = &attachments[0] else {
                panic!("canonical metadata should inspect successfully");
            };
            assert_eq!(metadata.alias, alias);
            assert_eq!(metadata.created_utc.as_deref(), timestamp);
            assert_eq!(block.end(), header);
            assert_eq!(block.start(), 0);
        }
    }

    #[test]
    fn wireguard_metadata_rejects_control_aliases_and_invalid_utc_dates() {
        assert_eq!(
            render_block("bad\n# wg-admin:end", None),
            Err(MetadataError::InvalidAlias)
        );
        for timestamp in [
            "2023-02-29T12:00:00Z",
            "2024-13-01T12:00:00Z",
            "2024-01-01T24:00:00Z",
            "2024-01-01T12:00:60Z",
            "2024-01-01T12:00:00+00:00",
        ] {
            assert_eq!(
                render_block("ok", Some(timestamp)),
                Err(MetadataError::InvalidTimestamp)
            );
        }
    }

    #[test]
    fn wireguard_metadata_reports_unsupported_and_misattached_blocks_redacted() {
        let source = "# wg-admin:begin\n# wg-admin:version=2\n# wg-admin:alias=b2s\n# wg-admin:end\n\n[Peer]\nPublicKey = AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=\n";
        let header = source.find("[Peer]").unwrap();
        let (attachments, issues) = inspect_document(source, &[header]);
        assert!(matches!(attachments[0], MetadataAttachment::Absent));
        assert!(
            issues
                .iter()
                .any(|issue| matches!(issue, MetadataIssue::Misattached { .. }))
        );
        assert!(
            issues
                .iter()
                .any(|issue| matches!(issue, MetadataIssue::UnsupportedVersion { .. }))
        );
        assert!(!format!("{issues:?}").contains("b2s"));
    }

    #[test]
    fn wireguard_metadata_missing_end_marker_still_attaches_as_invalid() {
        let source = "# wg-admin:begin\n# wg-admin:version=1\n# wg-admin:alias=b2s\n[Peer]\nPublicKey = AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=\n";
        let header = source.find("[Peer]").unwrap();
        let (attachments, _) = inspect_document(source, &[header]);
        assert!(matches!(
            attachments[0],
            MetadataAttachment::Invalid(MetadataIssue::MalformedBlock { .. })
        ));
    }

    #[test]
    fn wireguard_metadata_ignores_unrelated_admin_comments_and_rejects_duplicate_blocks() {
        let first = render_block("one", None).unwrap();
        let second = render_block("two", None).unwrap();
        let source = format!(
            "# administrator note: wg-admin is not mentioned as an owner\n{first}{second}[Peer]\n"
        );
        let header = source.find("[Peer]").unwrap();
        let (attachments, _) = inspect_document(&source, &[header]);
        assert!(matches!(
            attachments[0],
            MetadataAttachment::Invalid(MetadataIssue::DuplicateBlocks { count: 2, .. })
        ));
        let source = "# administrator note: wg-admin is not mentioned as an owner\n[Peer]\n";
        let header = source.find("[Peer]").unwrap();
        let (attachments, issues) = inspect_document(source, &[header]);
        assert!(matches!(attachments[0], MetadataAttachment::Absent));
        assert!(issues.is_empty());
    }
}