use std::{
    fmt::{self, Display},
    ops::Deref,
};

use data_encoding::BASE64;
use serde::{Deserialize, Serialize};

const SIGNATURE_LENGTH: usize = std::mem::size_of::<ed25519::SignatureBytes>();

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct Signature<S> {
    name: S,
    bytes: ed25519::SignatureBytes,
}

/// Type alias of a [Signature] using a `&str` as `name` field.
pub type SignatureRef<'a> = Signature<&'a str>;

/// Represents the signatures that Nix emits.
/// It consists of a name (an identifier for a public key), and an ed25519
/// signature (64 bytes).
/// It is generic over the string type that's used for the name, and there's
/// [SignatureRef] as a type alias for one containing &str.
impl<S> Signature<S>
where
    S: Deref<Target = str>,
{
    /// Constructs a new [Signature] from a name and public key.
    pub fn new(name: S, bytes: ed25519::SignatureBytes) -> Self {
        Self { name, bytes }
    }

    /// Parses a [Signature] from a string containing the name, a colon, and 64
    /// base64-encoded bytes (plus padding).
    /// These strings are commonly seen in the `Signature:` field of a NARInfo
    /// file.
    pub fn parse<'a>(input: &'a str) -> Result<Self, Error>
    where
        S: From<&'a str>,
    {
        let (name, bytes64) = input.split_once(':').ok_or(Error::MissingSeparator)?;

        if name.is_empty()
            || !name
                .chars()
                .all(|c| char::is_alphanumeric(c) || c == '-' || c == '.')
        {
            return Err(Error::InvalidName(name.to_string()));
        }

        if bytes64.len() != BASE64.encode_len(SIGNATURE_LENGTH) {
            return Err(Error::InvalidSignatureLen(bytes64.len()));
        }

        let mut bytes = [0; SIGNATURE_LENGTH];
        let mut buf = [0; SIGNATURE_LENGTH + 2];
        match BASE64.decode_mut(bytes64.as_bytes(), &mut buf) {
            Ok(SIGNATURE_LENGTH) => bytes.copy_from_slice(&buf[..SIGNATURE_LENGTH]),
            Ok(_) => unreachable!(),
            // keeping DecodePartial gets annoying lifetime-wise
            Err(_) => return Err(Error::DecodeError(input.to_string())),
        }

        Ok(Self {
            name: name.into(),
            bytes,
        })
    }

    /// Returns the name field of the signature.
    pub fn name(&self) -> &S {
        &self.name
    }

    /// Returns the 64 bytes of signatures.
    pub fn bytes(&self) -> &ed25519::SignatureBytes {
        &self.bytes
    }

    /// For a given fingerprint and ed25519 verifying key, ensure if the signature is valid.
    pub fn verify(&self, fingerprint: &[u8], verifying_key: &ed25519_dalek::VerifyingKey) -> bool {
        let signature = ed25519_dalek::Signature::from_bytes(self.bytes());

        verifying_key.verify_strict(fingerprint, &signature).is_ok()
    }

    /// Constructs a [SignatureRef] from this signature.
    pub fn as_ref(&self) -> SignatureRef<'_> {
        SignatureRef {
            name: self.name.deref(),
            bytes: self.bytes,
        }
    }
    pub fn to_owned(&self) -> Signature<String> {
        Signature {
            name: self.name.to_string(),
            bytes: self.bytes,
        }
    }
}

impl<'a, 'de, S> Deserialize<'de> for Signature<S>
where
    S: Deref<Target = str> + From<&'a str>,
    'de: 'a,
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        let str: &'de str = Deserialize::deserialize(deserializer)?;
        Self::parse(str).map_err(|_| {
            serde::de::Error::invalid_value(serde::de::Unexpected::Str(str), &"Signature")
        })
    }
}

impl<S: Display> Serialize for Signature<S>
where
    S: Deref<Target = str>,
{
    fn serialize<SR>(&self, serializer: SR) -> Result<SR::Ok, SR::Error>
    where
        SR: serde::Serializer,
    {
        let string: String = self.to_string();

        string.serialize(serializer)
    }
}

impl<S> Display for Signature<S>
where
    S: Display,
{
    fn fmt(&self, w: &mut fmt::Formatter) -> fmt::Result {
        write!(w, "{}:{}", self.name, BASE64.encode(&self.bytes))
    }
}

impl<S> std::hash::Hash for Signature<S>
where
    S: AsRef<str>,
{
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        state.write(self.name.as_ref().as_bytes());
        state.write(&self.bytes);
    }
}

#[derive(Debug, thiserror::Error, PartialEq, Eq)]
pub enum Error {
    #[error("Invalid name: {0}")]
    InvalidName(String),
    #[error("Missing separator")]
    MissingSeparator,
    #[error("Invalid signature len: (expected {} b64-encoded, got {}", BASE64.encode_len(SIGNATURE_LENGTH), .0)]
    InvalidSignatureLen(usize),
    #[error("Unable to base64-decode signature: {0}")]
    DecodeError(String),
}

#[cfg(test)]
mod test {
    use data_encoding::BASE64;
    use ed25519_dalek::VerifyingKey;
    use hex_literal::hex;
    use std::sync::LazyLock;

    use super::Signature;
    use rstest::rstest;

    const FINGERPRINT: &str = "1;/nix/store/syd87l2rxw8cbsxmxl853h0r6pdwhwjr-curl-7.82.0-bin;sha256:1b4sb93wp679q4zx9k1ignby1yna3z7c4c2ri3wphylbc2dwsys0;196040;/nix/store/0jqd0rlxzra1rs38rdxl43yh6rxchgc6-curl-7.82.0,/nix/store/6w8g7njm4mck5dmjxws0z1xnrxvl81xa-glibc-2.34-115,/nix/store/j5jxw3iy7bbz4a57fh9g2xm2gxmyal8h-zlib-1.2.12,/nix/store/yxvjs9drzsphm9pcf42a4byzj1kb9m7k-openssl-1.1.1n";

    /// The signing key labelled as `cache.nixos.org-1`,
    static PUB_CACHE_NIXOS_ORG_1: LazyLock<VerifyingKey> = LazyLock::new(|| {
        ed25519_dalek::VerifyingKey::from_bytes(
            BASE64
                .decode(b"6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=")
                .unwrap()[..]
                .try_into()
                .unwrap(),
        )
        .expect("embedded public key is valid")
    });

    #[rstest]
    #[case::valid_cache_nixos_org_1(&PUB_CACHE_NIXOS_ORG_1, &"cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==", FINGERPRINT, true)]
    #[case::valid_test1(&PUB_CACHE_NIXOS_ORG_1, &"cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==", FINGERPRINT, true)]
    #[case::valid_cache_nixos_org_different_name(&PUB_CACHE_NIXOS_ORG_1, &"cache.nixos.org-2:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==", FINGERPRINT, true)]
    #[case::fail_invalid_cache_nixos_org_1_signature(&PUB_CACHE_NIXOS_ORG_1, &"cache.nixos.org-1:TsTTb000000000000000000000000ytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==", FINGERPRINT, false)]
    #[case::fail_valid_sig_but_wrong_fp_cache_nixos_org_1(&PUB_CACHE_NIXOS_ORG_1, &"cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==", &FINGERPRINT[0..5], false)]
    fn verify_sigs(
        #[case] verifying_key: &VerifyingKey,
        #[case] sig_str: &'static str,
        #[case] fp: &str,
        #[case] expect_valid: bool,
    ) {
        let sig = Signature::<&str>::parse(sig_str).expect("must parse");
        assert_eq!(expect_valid, sig.verify(fp.as_bytes(), verifying_key));
    }

    #[rstest]
    #[case::wrong_length("cache.nixos.org-1:o1DTsjCz0PofLJ216P2RBuSulI8BAb6zHxWE4N+tzlcELk5Uk/GO2SCxWTRN5wJutLZZ+cHTMdWqOHF8")]
    #[case::wrong_name_newline("test\n:u01BybwQhyI5H1bW1EIWXssMDhDDIvXOG5uh8Qzgdyjz6U1qg6DHhMAvXZOUStIj6X5t4/ufFgR8i3fjf0bMAw==")]
    #[case::wrong_name_space("test :u01BybwQhyI5H1bW1EIWXssMDhDDIvXOG5uh8Qzgdyjz6U1qg6DHhMAvXZOUStIj6X5t4/ufFgR8i3fjf0bMAw==")]
    #[case::empty_name(
        ":u01BybwQhyI5H1bW1EIWXssMDhDDIvXOG5uh8Qzgdyjz6U1qg6DHhMAvXZOUStIj6X5t4/ufFgR8i3fjf0bMAw=="
    )]
    #[case::b64_only(
        "u01BybwQhyI5H1bW1EIWXssMDhDDIvXOG5uh8Qzgdyjz6U1qg6DHhMAvXZOUStIj6X5t4/ufFgR8i3fjf0bMAw=="
    )]
    fn parse_fail(#[case] input: &'static str) {
        Signature::<&str>::parse(input).expect_err("must fail");
    }

    #[test]
    fn serialize_deserialize() {
        let signature_actual = Signature {
            name: "cache.nixos.org-1",
            bytes: hex!(
                r#"4e c4 d3 6f 75 86 4d 92  a9 86 f6 1d 04 75 f0 a3
                   ac 1e 54 82 e6 4f 2b 54  8c b0 7e bd c5 fc f5 f3
                   a3 8d 18 9c 08 79 8a 03  84 42 3c c5 4b 92 3e 93
                   30 9e 06 31 7d c7 3d 55  91 74 3d 61 91 e2 99 05"#
            ),
        };
        let signature_str_json = "\"cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==\"";

        let serialized = serde_json::to_string(&signature_actual).expect("must serialize");
        assert_eq!(signature_str_json, &serialized);

        let deserialized: Signature<&str> =
            serde_json::from_str(signature_str_json).expect("must deserialize");
        assert_eq!(&signature_actual, &deserialized);
    }

    /// Construct a [Signature], using different String types for the name field.
    #[test]
    fn signature_owned() {
        let signature1 = Signature::<String>::parse("cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==").expect("must parse");
        let signature2 = Signature::<smol_str::SmolStr>::parse("cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==").expect("must parse");
        let signature3 = Signature::<&str>::parse("cache.nixos.org-1:TsTTb3WGTZKphvYdBHXwo6weVILmTytUjLB+vcX89fOjjRicCHmKA4RCPMVLkj6TMJ4GMX3HPVWRdD1hkeKZBQ==").expect("must parse");

        assert!(
            signature1.verify(FINGERPRINT.as_bytes(), &PUB_CACHE_NIXOS_ORG_1),
            "must verify"
        );
        assert!(
            signature2.verify(FINGERPRINT.as_bytes(), &PUB_CACHE_NIXOS_ORG_1),
            "must verify"
        );
        assert!(
            signature3.verify(FINGERPRINT.as_bytes(), &PUB_CACHE_NIXOS_ORG_1),
            "must verify"
        );
    }
}