Sign a request

use base64::prelude::*;
use ed25519_dalek::pkcs8::DecodePrivateKey;
use ed25519_dalek::{Signer, SigningKey};
use reqwest::blocking::{Client, Request};
use reqwest::{Method, Url};
use sha2::{Digest, Sha256};
use std::time::{SystemTime, UNIX_EPOCH};

/// An API key: the id the server looks up, and the
/// private half that signs.
struct Key {
    id: &'static str,
    signer: SigningKey,
}

impl Key {
    fn load(id: &'static str, pem: &str) -> anyhow::Result<Self> {
        let signer =
            SigningKey::read_pkcs8_pem_file(pem)?;
        Ok(Self { id, signer })
    }
}

/// Signs a built request over the method, path, query and
/// body it will send, so the two can never disagree.
trait Sign: Sized {
    fn sign(self, key: &Key) -> anyhow::Result<Self>;
}

impl Sign for Request {
    fn sign(mut self, key: &Key) -> anyhow::Result<Self> {
        let ts = SystemTime::now()
            .duration_since(UNIX_EPOCH)?
            .as_millis()
            .to_string();
        let body = self.body().and_then(|b| b.as_bytes());
        let body = body.unwrap_or_default();
        let canon = Canonical::new(
            &ts,
            self.method(),
            self.url(),
            body,
        );
        let sig = key.signer.sign(canon.0.as_bytes());
        let sig = BASE64_STANDARD.encode(sig.to_bytes());
        let headers = self.headers_mut();
        headers.insert("Novig-Key-Id", key.id.parse()?);
        headers.insert("Novig-Timestamp", ts.parse()?);
        headers.insert("Novig-Signature", sig.parse()?);
        Ok(self)
    }
}

/// The six NOVIG-V3 lines, joined by LF.
struct Canonical(String);

impl Canonical {
    fn new(
        ts: &str,
        method: &Method,
        url: &Url,
        body: &[u8],
    ) -> Self {
        let query =
            Query::from(url.query().unwrap_or(""));
        let hash = format!("{:x}", Sha256::digest(body));
        let method = method.as_str();
        let path = url.path();
        let lines = [
            "NOVIG-V3", ts, method, path, &query.0, &hash,
        ];
        Self(lines.join("\n"))
    }
}

/// Each part decoded and re-encoded, then sorted by
/// name and value.
struct Query(String);

impl From<&str> for Query {
    fn from(raw: &str) -> Self {
        let mut pairs = raw
            .split('&')
            .filter(|pair| !pair.is_empty())
            .map(|pair| {
                pair.split_once('=').unwrap_or((pair, ""))
            })
            .map(|(k, v)| {
                (Self::encode(k), Self::encode(v))
            })
            .collect::<Vec<_>>();
        pairs.sort();
        let pairs =
            pairs.iter().map(|(k, v)| format!("{k}={v}"));
        Self(pairs.collect::<Vec<_>>().join("&"))
    }
}

impl Query {
    fn encode(part: &str) -> String {
        Self::decode(part)
            .iter()
            .map(|&b| match b {
                b'-' | b'.' | b'_' | b'~' => {
                    (b as char).to_string()
                }
                _ if b.is_ascii_alphanumeric() => {
                    (b as char).to_string()
                }
                _ => format!("%{b:02X}"),
            })
            .collect()
    }

    /// Only `%XX` decodes. A bare `+` stays a `+`.
    fn decode(part: &str) -> Vec<u8> {
        let raw = part.as_bytes();
        let mut out = Vec::with_capacity(raw.len());
        let mut i = 0;
        while i < raw.len() {
            let escape =
                raw.get(i + 1..i + 3).and_then(Self::hex);
            match (raw[i], escape) {
                (b'%', Some(byte)) => {
                    out.push(byte);
                    i += 3;
                }
                (byte, _) => {
                    out.push(byte);
                    i += 1;
                }
            }
        }
        out
    }

    fn hex(pair: &[u8]) -> Option<u8> {
        let hi = (pair[0] as char).to_digit(16)?;
        let lo = (pair[1] as char).to_digit(16)?;
        Some((hi * 16 + lo) as u8)
    }
}