wintercount 0.1.0

Temporal policy provenance: pin the policy hash in force at every event, then answer 'what rules governed at time T' forever — named for the painted buffalo robes that recorded each year's defining event
Documentation 
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use wintercount::{from_ledger, policy_hash, PolicySet};

fn main() {
    // An audit ledger in Bad Apple format — every event pins the hash
    // of the policy bytes in force at write time.
    let v1 = policy_hash(b"allow: read\nrequire_approval: false");
    let v2 = policy_hash(b"allow: read\nrequire_approval: true");
    let ledger = vec![
        format!(r#"{{"ts":100,"type":"tool_call","data":{{"policy_hash":"{v1}","tool":"read_file"}}}}"#),
        format!(r#"{{"ts":150,"type":"tool_call","data":{{"policy_hash":"{v1}","tool":"write_file"}}}}"#),
        // policy.yaml edited at t=200 — the hash changes from here on.
        format!(r#"{{"ts":200,"type":"policy_reload","data":{{"policy_hash":"{v2}"}}}}"#),
        format!(r#"{{"ts":250,"type":"tool_call","data":{{"policy_hash":"{v2}","tool":"write_file"}}}}"#),
    ];

    let wc = from_ledger(ledger.into_iter());

    let mut registry = PolicySet::new();
    registry.register(b"allow: read\nrequire_approval: false".to_vec(), "policy-v1.yaml");
    registry.register(b"allow: read\nrequire_approval: true".to_vec(), "policy-v2.yaml");

    println!("the robe:");
    for m in wc.marks() {
        println!("  t={:4}  governed by {}  ({})", m.ts, &m.policy_hash[..12], m.label.as_deref().unwrap_or("-"));
    }

    println!("\ntransitions:");
    for (ts, old, new) in wc.transitions() {
        println!("  t={ts}: {} → {}", old.as_deref().unwrap_or("founding"), &new[..12]);
    }

    println!("\nwhat governed at t=175? {}", &wc.policy_at(175).unwrap()[..12]);
    println!("what governed at t=225? {}", &wc.policy_at(225).unwrap()[..12]);

    let foreign = wc.foreign_marks(&registry, true);
    println!("\nmarks under unrecognized policy: {}", foreign.len());
}