// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

package health

import (
	"git.ipng.ch/ipng/vpp-maglev/internal/config"
)

// ActivePoolIndex returns the priority-failover pool index for fe given
// the current backend states. The active pool is the first pool that
// contains at least one backend which is both in StateUp AND has a
// non-zero configured weight: a pool whose up backends are all
// weight=0 contributes no serving capacity, so failover falls through
// to the next tier. Returns 0 when no pool can serve, in which case
// every backend maps to weight 0 and the return value is unobservable.
//
// pool[0] is the primary, pool[1] the first fallback, and so on.
func ActivePoolIndex(fe config.Frontend, states map[string]State) int {
	for i, pool := range fe.Pools {
		for bName, pb := range pool.Backends {
			if states[bName] == StateUp && pb.Weight > 0 {
				return i
			}
		}
	}
	return 0
}

// BackendEffectiveWeight is the pure mapping from (pool index, active pool,
// backend state, config weight) to the desired VPP AS weight and flush hint.
// This is the single source of truth for the state → dataplane rule.
//
// A backend gets its configured weight iff it is up AND belongs to the
// currently-active pool. Every other case yields weight 0. Only StateDisabled
// produces flush=true (immediate session teardown).
//
//	state       in active pool   not in active pool    flush
//	--------    --------------   -------------------   -----
//	unknown     0                0                     no
//	up          configured       0 (standby)           no
//	down        0                0                     no
//	paused      0                0                     no
//	disabled    0                0                     yes
func BackendEffectiveWeight(poolIdx, activePool int, state State, cfgWeight int) (weight uint8, flush bool) {
	switch state {
	case StateUp:
		if poolIdx == activePool {
			return clampWeight(cfgWeight), false
		}
		return 0, false
	case StateDisabled:
		return 0, true
	default:
		return 0, false
	}
}

// EffectiveWeights computes per-pool per-backend effective weights for fe,
// given a snapshot of backend states. Result layout: weights[poolIdx][backendName].
func EffectiveWeights(fe config.Frontend, states map[string]State) map[int]map[string]uint8 {
	activePool := ActivePoolIndex(fe, states)
	out := make(map[int]map[string]uint8, len(fe.Pools))
	for poolIdx, pool := range fe.Pools {
		out[poolIdx] = make(map[string]uint8, len(pool.Backends))
		for bName, pb := range pool.Backends {
			w, _ := BackendEffectiveWeight(poolIdx, activePool, states[bName], pb.Weight)
			out[poolIdx][bName] = w
		}
	}
	return out
}

// ComputeFrontendState derives the FrontendState for fe from a snapshot of
// backend states. Rules:
//
//   - no backends → unknown
//   - every referenced backend is in StateUnknown → unknown
//   - any backend has effective weight > 0 → up
//   - otherwise → down
func ComputeFrontendState(fe config.Frontend, states map[string]State) FrontendState {
	// Unique set of backends referenced by this frontend (a single backend
	// may appear in multiple pools; we count it once).
	seen := make(map[string]struct{})
	for _, pool := range fe.Pools {
		for bName := range pool.Backends {
			seen[bName] = struct{}{}
		}
	}
	if len(seen) == 0 {
		return FrontendStateUnknown
	}
	allUnknown := true
	for bName := range seen {
		if states[bName] != StateUnknown {
			allUnknown = false
			break
		}
	}
	if allUnknown {
		return FrontendStateUnknown
	}
	ew := EffectiveWeights(fe, states)
	for _, poolMap := range ew {
		for _, w := range poolMap {
			if w > 0 {
				return FrontendStateUp
			}
		}
	}
	return FrontendStateDown
}

func clampWeight(w int) uint8 {
	if w < 0 {
		return 0
	}
	if w > 100 {
		return 100
	}
	return uint8(w)
}