use std::fmt;

use jacquard_common::types::string::AtUri;
use unicode_normalization::UnicodeNormalization;

/// Build the visible path for a Part membership.
///
/// A bare Part name retains the historical `models/` root. Once the author
/// supplies a relative path, it is preserved as-is (after `CanonicalPath`
/// validation). Parts owned by another authority are put below an explicit
/// source namespace. Jacquard validates the URI, collection, and rkey before
/// this function reaches the string/SQL boundary.
pub fn project_part_path(
    project: &AtUri,
    part: &AtUri,
    declared_name: &str,
) -> Result<CanonicalPath, PathDiagnostic> {
    if project.authority().as_str() == part.authority().as_str() {
        let declared = if declared_name.contains('/') {
            declared_name.to_owned()
        } else {
            format!("models/{declared_name}")
        };
        return CanonicalPath::parse(&declared);
    }

    let Some(collection) = part.collection() else {
        return Err(PathDiagnostic::SourceIdentity);
    };
    let Some(rkey) = part.rkey() else {
        return Err(PathDiagnostic::SourceIdentity);
    };
    if collection.as_str() != "space.polymodel.library.part" {
        return Err(PathDiagnostic::SourceIdentity);
    }
    let path = format!(
        "external/{}/{}",
        encode_identity_segment(part.authority().as_str()),
        encode_identity_segment(rkey.as_str()),
    );
    CanonicalPath::parse(&path)
}

/// Decode an external Part path into its authority and rkey.
///
/// The rkey is deliberately returned unchanged: Jacquard's validated rkey is
/// already a safe relative path segment. This is a pure inverse of the
/// authority encoding and is useful for route/path round-trip tests.
#[cfg(test)]
pub fn decode_external_part_path(path: &CanonicalPath) -> Option<(String, String)> {
    let segments = path.segments().collect::<Vec<_>>();
    if segments.first().copied()? != "external" || segments.len() != 3 {
        return None;
    }
    let authority = decode_identity_segment(segments[1])?;
    let rkey = decode_identity_segment(segments[2])?;
    jacquard_common::types::recordkey::Rkey::<String>::new_owned(rkey.clone()).ok()?;
    Some((authority, rkey))
}

/// Escape one authority component with a self-delimiting, ASCII-only codec.
///
/// `x` is the only prefix and `_hh` is the only escape form. The unescaped
/// alphabet excludes `_`, `%`, `.`, and path separators, so escaped-looking
/// input, dot segments, Unicode normalization, and URL decoding cannot alias
/// an encoded identity.
fn encode_identity_segment(value: &str) -> String {
    let mut out = String::from("x");
    for byte in value.as_bytes() {
        if byte.is_ascii_alphanumeric() || *byte == b'-' || *byte == b'~' {
            out.push(*byte as char);
        } else {
            out.push('_');
            out.push(hex_digit(byte >> 4));
            out.push(hex_digit(byte & 0x0f));
        }
    }
    out
}

#[cfg(test)]
fn decode_identity_segment(value: &str) -> Option<String> {
    let bytes = value.strip_prefix('x')?.as_bytes();
    let mut decoded = Vec::with_capacity(bytes.len());
    let mut index = 0;
    while index < bytes.len() {
        match bytes[index] {
            b'_' if index + 2 < bytes.len() => {
                let high = from_hex(bytes[index + 1])?;
                let low = from_hex(bytes[index + 2])?;
                decoded.push(high << 4 | low);
                index += 3;
            }
            byte if byte.is_ascii_alphanumeric() || byte == b'-' || byte == b'~' => {
                decoded.push(byte);
                index += 1;
            }
            _ => return None,
        }
    }
    String::from_utf8(decoded).ok()
}

fn hex_digit(value: u8) -> char {
    char::from(b"0123456789abcdef"[value as usize])
}

#[cfg(test)]
fn from_hex(value: u8) -> Option<u8> {
    match value {
        b'0'..=b'9' => Some(value - b'0'),
        b'a'..=b'f' => Some(value - b'a' + 10),
        _ => None,
    }
}

/// A validated, NFC-preserving virtual filename. It is never interpreted as a URL.
#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct CanonicalPath(String);

impl CanonicalPath {
    pub fn parse(input: &str) -> Result<Self, PathDiagnostic> {
        if input.is_empty() || input.starts_with('/') || input.ends_with('/') {
            return Err(PathDiagnostic::Slash);
        }
        if input.contains('%') {
            return Err(PathDiagnostic::Percent);
        }
        if input.contains('\\') {
            return Err(PathDiagnostic::Backslash);
        }
        if input.contains('?') || input.contains('#') {
            return Err(PathDiagnostic::QueryOrFragment);
        }
        if input.chars().any(|c| c.is_control() || c == ':') {
            return Err(PathDiagnostic::ControlOrColon);
        }
        if input.nfc().collect::<String>() != input {
            return Err(PathDiagnostic::NotNfc);
        }
        for segment in input.split('/') {
            if segment.is_empty() || segment == "." || segment == ".." {
                return Err(PathDiagnostic::Segment);
            }
        }
        Ok(Self(input.to_owned()))
    }

    pub fn as_str(&self) -> &str {
        &self.0
    }

    pub fn segments(&self) -> impl Iterator<Item = &str> {
        self.0.split('/')
    }
}

impl fmt::Display for CanonicalPath {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.0.fmt(f)
    }
}

#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub enum RootId {
    Mpd,
    Manifest,
    Models,
    Parts,
    P,
    External,
    OfficialParts,
    OfficialP,
}

impl RootId {
    pub fn as_str(self) -> &'static str {
        match self {
            Self::Mpd => "mpd",
            Self::Manifest => "manifest",
            Self::Models => "models",
            Self::Parts => "parts",
            Self::P => "p",
            Self::External => "external",
            Self::OfficialParts => "official-parts",
            Self::OfficialP => "official-p",
        }
    }

    pub fn case_insensitive(self) -> bool {
        !matches!(self, Self::Mpd | Self::Manifest | Self::External)
    }
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum PathDiagnostic {
    Slash,
    Percent,
    Backslash,
    QueryOrFragment,
    ControlOrColon,
    NotNfc,
    Segment,
    SourceIdentity,
}

#[derive(Clone, Debug, Eq, Hash, PartialEq)]
pub struct LookupEntry {
    pub root: RootId,
    pub path: CanonicalPath,
    pub target: String,
}

/// Resolve according to PM-75's fixed source precedence.
/// The maps contain already validated entries and only perform filename lookup.
pub fn resolve_precedence(
    path: &CanonicalPath,
    mpd: &[LookupEntry],
    manifest: &[LookupEntry],
    uploaded: &[LookupEntry],
    official: &[LookupEntry],
) -> Option<LookupEntry> {
    exact(path, mpd)
        .or_else(|| exact(path, manifest))
        .or_else(|| {
            roots(
                path,
                uploaded,
                &[RootId::Models, RootId::Parts, RootId::P, RootId::External],
            )
        })
        .or_else(|| roots(path, official, &[RootId::OfficialParts, RootId::OfficialP]))
}

fn exact(path: &CanonicalPath, entries: &[LookupEntry]) -> Option<LookupEntry> {
    entries.iter().find(|e| e.path == *path).cloned()
}

fn roots(path: &CanonicalPath, entries: &[LookupEntry], roots: &[RootId]) -> Option<LookupEntry> {
    roots.iter().find_map(|root| {
        entries
            .iter()
            .filter(|e| e.root == *root)
            .find(|e| {
                e.path == *path
                    || (root.case_insensitive()
                        && e.path.as_str().eq_ignore_ascii_case(path.as_str()))
            })
            .cloned()
    })
}

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

    #[test]
    fn rejects_unsafe_and_non_nfc_paths() {
        for value in [
            "",
            "/a",
            "a/",
            "a//b",
            "a/../b",
            "a/./b",
            "a%2Fb",
            "a\\b",
            "a:b",
            "a?b",
            "a#b",
            "a\u{0000}b",
        ] {
            assert!(CanonicalPath::parse(value).is_err(), "{value:?}");
        }
        assert!(CanonicalPath::parse("cafe\u{301}").is_err());
        assert_eq!(CanonicalPath::parse("café").unwrap().as_str(), "café");
    }

    #[test]
    fn authority_path_codec_round_trips_and_is_injective() {
        let project =
            AtUri::new_owned("at://did:plc:owner/space.polymodel.library.thing/project").unwrap();
        let mut paths = Vec::new();
        for (authority, rkey) in [
            ("did:plc:alpha", "same_rkey"),
            ("did:plc:beta", "same_rkey"),
            ("did:plc:alpha", "same-rkey"),
            ("did:plc:alpha", "a.b"),
            ("did:plc:alpha", "a~b:c"),
        ] {
            let part = AtUri::new_owned(format!(
                "at://{authority}/space.polymodel.library.part/{rkey}"
            ))
            .unwrap();
            let path = project_part_path(&project, &part, "ignored.ldr").unwrap();
            assert!(path.as_str().starts_with("external/x"));
            assert!(!path.as_str().starts_with('/'));
            assert!(!path.as_str().contains(".."));
            let (decoded_authority, decoded_rkey) = decode_external_part_path(&path).unwrap();
            assert_eq!(decoded_authority, authority);
            assert_eq!(decoded_rkey, rkey);
            paths.push(path);
        }
        for (index, path) in paths.iter().enumerate() {
            assert!(paths[index + 1..].iter().all(|other| other != path));
        }
    }

    #[test]
    fn same_authority_preserves_declared_project_relative_path() {
        let project =
            AtUri::new_owned("at://did:plc:owner/space.polymodel.library.thing/project").unwrap();
        let part =
            AtUri::new_owned("at://did:plc:owner/space.polymodel.library.part/part").unwrap();
        assert_eq!(
            project_part_path(&project, &part, "models/sub/main.ldr")
                .unwrap()
                .as_str(),
            "models/sub/main.ldr"
        );
        assert_eq!(
            project_part_path(&project, &part, "main.ldr")
                .unwrap()
                .as_str(),
            "models/main.ldr"
        );
    }

    #[test]
    fn external_codec_rejects_path_aliases_and_invalid_rkeys() {
        let project =
            AtUri::new_owned("at://did:plc:owner/space.polymodel.library.thing/project").unwrap();
        let part =
            AtUri::new_owned("at://did:plc:external/space.polymodel.library.part/x").unwrap();
        let valid = project_part_path(&project, &part, "x.ldr").unwrap();
        assert_eq!(
            decode_external_part_path(&valid).unwrap(),
            ("did:plc:external".to_owned(), "x".to_owned())
        );
        let malformed_escape = CanonicalPath::parse("external/x_zz/x").unwrap();
        assert!(decode_external_part_path(&malformed_escape).is_none());
        assert!(CanonicalPath::parse("external/xdid_3aplc/a%2Fb").is_err());
        assert!(CanonicalPath::parse("external/xdid_3aplc/../x").is_err());
        assert!(
            AtUri::<String>::new_owned(
                "at://did:plc:owner/space.polymodel.library.part/cafe\u{301}"
            )
            .is_err()
        );
    }

    #[test]
    fn precedence_and_ascii_case_policy_are_stable() {
        let path = CanonicalPath::parse("parts/3001.dat").unwrap();
        let entry = |root, target: &str| LookupEntry {
            root,
            path: path.clone(),
            target: target.into(),
        };
        let found =
            resolve_precedence(&path, &[], &[], &[entry(RootId::Models, "model")], &[]).unwrap();
        assert_eq!(found.target, "model");
        let upper = CanonicalPath::parse("PARTS/3001.DAT").unwrap();
        assert!(
            resolve_precedence(&upper, &[], &[], &[entry(RootId::Models, "model")], &[]).is_some()
        );
        let mpd = entry(RootId::Mpd, "mpd");
        assert_eq!(
            resolve_precedence(&path, &[mpd], &[], &[], &[])
                .unwrap()
                .target,
            "mpd"
        );
    }
}