//! Per-agent CPU/memory limit overrides. Stored at
//! `/var/lib/hyperhive/meta/resource-limits.json` alongside
//! `topology.json`, `tool-groups.json` and `capabilities.json`.
//!
//! Format: a JSON object mapping agent name to an object with optional
//! `cpu_quota` / `memory_max` strings, passed verbatim to systemd's
//! `CPUQuota=` / `MemoryMax=` in the per-container drop-in:
//!
//! ```json
//! {
//!   "sock": { "cpu_quota": "400%", "memory_max": "8G" }
//! }
//! ```
//!
//! Fallback is **per field**: an absent file, an absent agent, or an
//! absent field all fall back to the hive-wide
//! `services.hyperhive.agentCpuQuota` / `agentMemoryMax`. So an agent
//! can raise only its memory cap and keep tracking the hive default for
//! CPU — see [`effective`].
//!
//! Read path: `lifecycle::host_config::write_dropins`, on every spawn
//! and every rebuild.
//!
//! Why host-side JSON and not an option in the agent's own `agent.nix`:
//! the drop-in lands on the *host's* `container@h-<name>.service`, so
//! c0re would have to `nix eval` the agent's whole nixosConfiguration
//! just to read two strings. It is also the wrong trust boundary —
//! a resource *cap* should not be sourced from the capped party.

use std::collections::BTreeMap;
use std::path::PathBuf;

const RESOURCE_LIMITS_FILE: &str = "resource-limits.json";

/// One agent's overrides. Both fields optional and independent; `None`
/// means "use the hive-wide default for this field".
#[derive(Debug, Clone, Default, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
pub struct AgentLimits {
    /// systemd `CPUQuota=` value, e.g. `"400%"`.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub cpu_quota: Option<String>,
    /// systemd `MemoryMax=` value, e.g. `"8G"`.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub memory_max: Option<String>,
}

impl AgentLimits {
    /// True when neither field is set — such an entry is dropped rather
    /// than persisted as an empty object.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.cpu_quota.is_none() && self.memory_max.is_none()
    }
}

#[must_use]
pub fn resource_limits_path() -> PathBuf {
    crate::paths::meta_root().join(RESOURCE_LIMITS_FILE)
}

/// Read the per-agent limit map. Returns an empty map when the file is
/// absent or unparsable — callers treat a missing entry as "hive-wide
/// defaults", which is also the safe failure mode for a malformed file.
#[must_use]
pub fn read() -> BTreeMap<String, AgentLimits> {
    let path = resource_limits_path();
    let Ok(raw) = std::fs::read_to_string(&path) else {
        return BTreeMap::new();
    };
    serde_json::from_str(&raw).unwrap_or_default()
}

/// Resolve the effective values for an agent, filling each unset field
/// from the hive-wide default. This is the single place the fallback
/// rule lives; `write_dropins` calls it and passes the result straight
/// to systemd.
///
/// Reads the override file. Use [`effective_from`] when resolving more
/// than one agent in a row.
#[must_use]
pub fn effective(name: &str, hive_cpu_quota: &str, hive_memory_max: &str) -> (String, String) {
    effective_from(&read(), name, hive_cpu_quota, hive_memory_max)
}

/// [`effective`] against an already-loaded map — the multi-agent form.
///
/// `container_view::build_all` renders every agent on each SSE scan, so
/// it loads the map once and calls this per agent rather than re-reading
/// the same small file N times per scan.
#[must_use]
pub fn effective_from(
    limits: &BTreeMap<String, AgentLimits>,
    name: &str,
    hive_cpu_quota: &str,
    hive_memory_max: &str,
) -> (String, String) {
    match limits.get(name) {
        Some(l) => resolve(l, hive_cpu_quota, hive_memory_max),
        None => (hive_cpu_quota.to_owned(), hive_memory_max.to_owned()),
    }
}

/// Pure core of [`effective`], split out so the fallback matrix is
/// testable without touching the filesystem.
#[must_use]
fn resolve(limits: &AgentLimits, hive_cpu_quota: &str, hive_memory_max: &str) -> (String, String) {
    let cpu = limits
        .cpu_quota
        .clone()
        .unwrap_or_else(|| hive_cpu_quota.to_owned());
    let mem = limits
        .memory_max
        .clone()
        .unwrap_or_else(|| hive_memory_max.to_owned());
    (cpu, mem)
}

/// Effective `MemoryMax=` string for `name` against an already-loaded
/// map — the memory-only half of [`effective_from`], split out because
/// [`effective_memory_bytes_from`] doesn't have a CPU quota to pass.
/// No single-agent convenience wrapper (unlike [`effective`] /
/// [`effective_from`]): every current caller already has a loaded map
/// in hand, so one would just be dead code.
#[must_use]
fn effective_memory_max_from(
    limits: &BTreeMap<String, AgentLimits>,
    name: &str,
    hive_memory_max: &str,
) -> String {
    limits
        .get(name)
        .and_then(|l| l.memory_max.clone())
        .unwrap_or_else(|| hive_memory_max.to_owned())
}

/// Effective `MemoryMax=` for `name`, as a raw byte count, against an
/// already-loaded map — the form `render_flake_with_lookup` uses so it
/// doesn't re-read `resource-limits.json` once per agent (same
/// reasoning as [`effective_from`] / `container_view::build_all`). No
/// single-agent convenience wrapper, for the same reason as
/// [`effective_memory_max_from`] — every current caller already has a
/// loaded map in hand.
///
/// Logs a `warn!` naming `name` when the effective value is a RAM
/// percentage: unlike `"infinity"`, a percentage IS a real, resolvable
/// cap — resolving one just needs host `MemTotal`, which this module
/// doesn't track — so today it degrades to `None` (no derived heap
/// ceiling) rather than silently guessing wrong against `MemTotal`.
#[must_use]
pub fn effective_memory_bytes_from(
    limits: &BTreeMap<String, AgentLimits>,
    name: &str,
    hive_memory_max: &str,
) -> Option<u64> {
    let mem = effective_memory_max_from(limits, name, hive_memory_max);
    if mem.ends_with('%') {
        tracing::warn!(
            agent = name,
            memory_max = %mem,
            "effective MemoryMax= is a RAM percentage; can't derive a JSC heap ceiling from it \
             without host MemTotal — leaving BUN_JSC_forceRAMSize unset for this agent"
        );
    }
    parse_bytes(&mem)
}

/// Parse a systemd `MemoryMax=`-style byte-size value (`"4G"`, `"512M"`,
/// a bare byte count, optionally with a decimal like `"1.5G"`) into a
/// raw byte count. systemd's `K`/`M`/`G`/`T` suffixes are IEC binary
/// (1024-based), not decimal — this matches. Pure integer arithmetic
/// throughout (via `u128` headroom) rather than `f64`: a `MemoryMax=`
/// value is always a small non-negative decimal (enforced by
/// [`is_plain_number`] upstream in [`validate_memory_max`]), so floats
/// would only add rounding / sign-loss risk for no benefit. Returns
/// `None` for `"infinity"` and percentages; see
/// [`effective_memory_bytes_from`] for why those can't be turned into a
/// byte count here.
#[must_use]
pub fn parse_bytes(value: &str) -> Option<u64> {
    if value == "infinity" || value.ends_with('%') {
        return None;
    }
    let (mantissa, exponent) = [('K', 1u32), ('M', 2), ('G', 3), ('T', 4)]
        .into_iter()
        .find_map(|(suffix, exp)| {
            value
                .strip_suffix([suffix, suffix.to_ascii_lowercase()])
                .map(|m| (m, exp))
        })
        .unwrap_or((value, 0));
    let scale = u128::from(1024u64.checked_pow(exponent)?);
    let (int_part, frac_part) = mantissa.split_once('.').unwrap_or((mantissa, ""));
    let whole: u128 = int_part.parse().ok()?;
    let mut bytes = whole.checked_mul(scale)?;
    if !frac_part.is_empty() {
        let frac_num: u128 = frac_part.parse().ok()?;
        let frac_denom = 10u128.checked_pow(u32::try_from(frac_part.len()).ok()?)?;
        bytes = bytes.checked_add(frac_num.checked_mul(scale)? / frac_denom)?;
    }
    u64::try_from(bytes).ok()
}

/// Persist the full map. Sorted JSON output keeps meta-repo diffs
/// minimal.
///
/// # Errors
///
/// Returns an `io::Error` when the meta dir can't be created or the
/// file can't be written (permissions, disk full). Serialization
/// failure is surfaced as `InvalidData`, though it can't happen for
/// this type — it's a plain map of strings.
pub fn write(map: &BTreeMap<String, AgentLimits>) -> std::io::Result<()> {
    let path = resource_limits_path();
    if let Some(parent) = path.parent() {
        std::fs::create_dir_all(parent)?;
    }
    let text = serde_json::to_string_pretty(map)
        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))?;
    std::fs::write(&path, format!("{text}\n"))
}

/// Set one agent's overrides and persist. An entry with both fields
/// unset is removed rather than stored, so "reset to hive defaults" and
/// "never configured" are the same state on disk.
///
/// # Errors
///
/// Propagates whatever the `write` function fails with. An unreadable or malformed
/// existing file is *not* an error — the `read` function degrades to an empty map,
/// so this call rewrites the file from scratch.
pub fn set_limits(name: &str, limits: &AgentLimits) -> std::io::Result<()> {
    let mut current = read();
    if limits.is_empty() {
        current.remove(name);
    } else {
        current.insert(name.to_owned(), limits.clone());
    }
    write(&current)
}

/// Validate a systemd `CPUQuota=` value. Percentages only, and values
/// above 100% are legitimate (one full core is 100%).
///
/// Validated because the value is written verbatim into a drop-in: a
/// typo makes systemd reject the unit, which means the container stops
/// starting at all. Better to refuse at the CLI than to brick a spawn.
///
/// # Errors
///
/// Returns a human-readable message naming the offending value when it
/// isn't a percentage. The string is surfaced straight to the operator,
/// so it names the expected shape rather than just saying "invalid".
pub fn validate_cpu_quota(value: &str) -> Result<(), String> {
    if is_percentage(value) {
        return Ok(());
    }
    Err(format!(
        "invalid CPUQuota {value:?}: expected a percentage such as \"200%\" \
         (100% = one full core)"
    ))
}

/// Validate a systemd `MemoryMax=` value: a byte count with an optional
/// `K`/`M`/`G`/`T` suffix, a percentage of physical memory, or the
/// literal `infinity`.
///
/// # Errors
///
/// Returns a human-readable message naming the offending value and the
/// three accepted shapes. Same operator-facing contract as
/// [`validate_cpu_quota`].
pub fn validate_memory_max(value: &str) -> Result<(), String> {
    if value == "infinity" || is_percentage(value) || is_byte_size(value) {
        return Ok(());
    }
    Err(format!(
        "invalid MemoryMax {value:?}: expected a size such as \"8G\", a percentage \
         such as \"50%\", or \"infinity\""
    ))
}

/// A decimal number followed by `%`.
fn is_percentage(value: &str) -> bool {
    value.strip_suffix('%').is_some_and(is_plain_number)
}

/// A decimal number with an optional single-letter binary suffix.
fn is_byte_size(value: &str) -> bool {
    let mantissa = value
        .strip_suffix(['K', 'M', 'G', 'T', 'k', 'm', 'g', 't'])
        .unwrap_or(value);
    is_plain_number(mantissa)
}

/// Digits, optionally followed by a single `.` and more digits. Hand
/// rolled rather than pulling in a regex dependency for two patterns;
/// deliberately rejects the exponent/sign forms `f64::from_str` accepts,
/// since systemd wouldn't take them either.
fn is_plain_number(value: &str) -> bool {
    let mut parts = value.splitn(2, '.');
    let int = parts.next().unwrap_or_default();
    if int.is_empty() || !int.bytes().all(|b| b.is_ascii_digit()) {
        return false;
    }
    match parts.next() {
        None => true,
        Some(frac) => !frac.is_empty() && frac.bytes().all(|b| b.is_ascii_digit()),
    }
}

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

    const HIVE_CPU: &str = "200%";
    const HIVE_MEM: &str = "4G";

    fn limits(cpu: Option<&str>, mem: Option<&str>) -> AgentLimits {
        AgentLimits {
            cpu_quota: cpu.map(ToOwned::to_owned),
            memory_max: mem.map(ToOwned::to_owned),
        }
    }

    #[test]
    fn unset_agent_gets_hive_defaults() {
        let (cpu, mem) = resolve(&AgentLimits::default(), HIVE_CPU, HIVE_MEM);
        assert_eq!(cpu, "200%");
        assert_eq!(mem, "4G");
    }

    #[test]
    fn both_fields_override() {
        let (cpu, mem) = resolve(&limits(Some("400%"), Some("8G")), HIVE_CPU, HIVE_MEM);
        assert_eq!(cpu, "400%");
        assert_eq!(mem, "8G");
    }

    /// The point of per-field fallback: overriding memory must not drag
    /// CPU along with it.
    #[test]
    fn partial_override_keeps_other_field_on_hive_default() {
        let (cpu, mem) = resolve(&limits(None, Some("8G")), HIVE_CPU, HIVE_MEM);
        assert_eq!(cpu, "200%", "cpu should still track the hive default");
        assert_eq!(mem, "8G");

        let (cpu, mem) = resolve(&limits(Some("400%"), None), HIVE_CPU, HIVE_MEM);
        assert_eq!(cpu, "400%");
        assert_eq!(mem, "4G", "memory should still track the hive default");
    }

    #[test]
    fn empty_entry_is_reported_empty() {
        assert!(AgentLimits::default().is_empty());
        assert!(!limits(None, Some("8G")).is_empty());
        assert!(!limits(Some("400%"), None).is_empty());
    }

    #[test]
    fn malformed_json_reads_as_empty_map() {
        let parsed: BTreeMap<String, AgentLimits> =
            serde_json::from_str("{ not json").unwrap_or_default();
        assert!(parsed.is_empty());
    }

    /// Absent fields must deserialize to `None`, not fail — an entry
    /// written by an older version with only one field must still load.
    #[test]
    fn partial_entry_deserializes() {
        let map: BTreeMap<String, AgentLimits> =
            serde_json::from_str(r#"{"sock":{"memory_max":"8G"}}"#).expect("parses");
        assert_eq!(map["sock"], limits(None, Some("8G")));
    }

    #[test]
    fn empty_fields_are_not_serialized() {
        let map = BTreeMap::from([("sock".to_owned(), limits(None, Some("8G")))]);
        let text = serde_json::to_string(&map).expect("serializes");
        assert_eq!(text, r#"{"sock":{"memory_max":"8G"}}"#);
    }

    #[test]
    fn accepts_valid_cpu_quotas() {
        for v in ["100%", "200%", "400%", "50%", "12.5%"] {
            assert!(validate_cpu_quota(v).is_ok(), "{v} should be valid");
        }
    }

    #[test]
    fn rejects_invalid_cpu_quotas() {
        for v in ["", "200", "%", "abc", "200%%", "-50%", "2e2%", "200 %"] {
            assert!(validate_cpu_quota(v).is_err(), "{v} should be rejected");
        }
    }

    #[test]
    fn accepts_valid_memory_maxes() {
        for v in [
            "8G", "512M", "1024", "2T", "4096K", "50%", "infinity", "1.5G",
        ] {
            assert!(validate_memory_max(v).is_ok(), "{v} should be valid");
        }
    }

    #[test]
    fn rejects_invalid_memory_maxes() {
        for v in ["", "8GB", "G", "abc", "-8G", "8 G", "Infinity", "8Gi"] {
            assert!(validate_memory_max(v).is_err(), "{v} should be rejected");
        }
    }

    #[test]
    fn parse_bytes_handles_plain_and_suffixed_values() {
        assert_eq!(parse_bytes("1024"), Some(1024));
        assert_eq!(parse_bytes("4G"), Some(4 * 1024 * 1024 * 1024));
        assert_eq!(parse_bytes("512M"), Some(512 * 1024 * 1024));
        assert_eq!(parse_bytes("4096K"), Some(4096 * 1024));
        assert_eq!(parse_bytes("2T"), Some(2 * 1024 * 1024 * 1024 * 1024));
        assert_eq!(
            parse_bytes("1.5G"),
            Some(1024 * 1024 * 1024 + 512 * 1024 * 1024)
        );
        // Lowercase suffixes accepted, matching `is_byte_size`.
        assert_eq!(parse_bytes("4g"), Some(4 * 1024 * 1024 * 1024));
    }

    #[test]
    fn parse_bytes_rejects_infinity_and_percentages() {
        assert_eq!(parse_bytes("infinity"), None);
        assert_eq!(parse_bytes("50%"), None);
    }

    #[test]
    fn parse_bytes_rejects_garbage() {
        for v in ["", "abc", "-8G", "8Gi"] {
            assert_eq!(parse_bytes(v), None, "{v} should not parse");
        }
    }

    #[test]
    fn effective_memory_bytes_falls_back_to_hive_default() {
        // Empty map, i.e. no per-agent override — exercises the
        // "use the hive-wide default" arm end to end.
        let empty = BTreeMap::new();
        assert_eq!(
            effective_memory_bytes_from(&empty, "nobody-configured-this-agent", "4G"),
            Some(4 * 1024 * 1024 * 1024)
        );
        assert_eq!(
            effective_memory_bytes_from(&empty, "nobody-configured-this-agent", "infinity"),
            None
        );
    }

    #[test]
    fn effective_memory_bytes_from_prefers_per_agent_override() {
        let map = BTreeMap::from([("sock".to_owned(), limits(None, Some("8G")))]);
        assert_eq!(
            effective_memory_bytes_from(&map, "sock", HIVE_MEM),
            Some(8 * 1024 * 1024 * 1024)
        );
        // A different agent not in the map still falls back to the
        // hive-wide default from the same loaded map (no re-read).
        assert_eq!(
            effective_memory_bytes_from(&map, "iris", HIVE_MEM),
            Some(4 * 1024 * 1024 * 1024)
        );
    }

    /// A percentage cap is real and resolvable in principle, but this
    /// module has no host `MemTotal` to resolve it against — must
    /// degrade to `None` (no derived heap ceiling) rather than silently
    /// treating it as unbounded or guessing a number. `warn!` firing is
    /// exercised for coverage but not asserted on (no tracing test
    /// subscriber wired up here) — the `None` return is the contract.
    #[test]
    fn effective_memory_bytes_from_returns_none_for_percentage() {
        let map = BTreeMap::from([("sock".to_owned(), limits(None, Some("50%")))]);
        assert_eq!(effective_memory_bytes_from(&map, "sock", HIVE_MEM), None);
    }
}