package majordomo

import (
	"context"
	"errors"
	"fmt"

	"gitea.stevedudenhoeffer.com/steve/majordomo/health"
	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
)

// ErrChainExhausted reports that every element of a failover chain failed
// (or was skipped while backed off). It is always joined with the
// per-target errors.
var ErrChainExhausted = errors.New("all chain targets failed")

// chainTarget is one resolved element of a failover chain.
type chainTarget struct {
	// key identifies the target for health tracking: "provider/model-id".
	key   string
	model llm.Model
}

// chain implements llm.Model over an ordered list of targets with
// health-tracked failover. A single-element spec is a chain of one — the
// behavior (retry-on-transient, backoff bookkeeping) is identical, so
// callers never branch on what Parse returned.
//
// Semantics (ADR-0006, ADR-0008):
//   - Targets are tried head-to-tail; targets currently backed off are
//     skipped.
//   - A transient error is retried on the same target (ChainConfig
//     TransientRetries, default 1). Every failed attempt counts toward the
//     target's consecutive-failure threshold; when the tracker benches the
//     target (default: 2 consecutive transient failures → exponential
//     capped cooldown) the chain stops retrying it and advances.
//   - Model-not-found advances without penalizing health. Other permanent
//     errors fail fast by default (AdvanceOnPermanent flips this).
//   - Any success resets the target's health.
//   - When every target fails or is skipped, the returned error joins
//     ErrChainExhausted with each target's reason.
type chain struct {
	targets []chainTarget
	tracker *health.Tracker
	cfg     ChainConfig
}

// Targets returns the resolved "provider/model" keys in chain order
// (diagnostics and tests).
func (c *chain) Targets() []string {
	keys := make([]string, len(c.targets))
	for i, t := range c.targets {
		keys[i] = t.key
	}
	return keys
}

// Capabilities reports the head element's capabilities — the chain's
// preferred target (ADR-0008). Per-attempt media normalization uses the
// actual target's capabilities, not this value.
func (c *chain) Capabilities() llm.Capabilities {
	return c.targets[0].model.Capabilities()
}

// Generate tries each target per the chain semantics above.
func (c *chain) Generate(ctx context.Context, req llm.Request, opts ...llm.Option) (*llm.Response, error) {
	req = req.Apply(opts...)
	return chainDo(ctx, c, func(ctx context.Context, t chainTarget) (*llm.Response, error) {
		return t.model.Generate(ctx, req)
	})
}

// Stream tries each target per the chain semantics. Failover applies to
// establishing the stream; once a stream is open, mid-stream errors
// propagate to the consumer rather than restarting on another target
// (replaying half-delivered output would duplicate content).
func (c *chain) Stream(ctx context.Context, req llm.Request, opts ...llm.Option) (llm.Stream, error) {
	req = req.Apply(opts...)
	return chainDo(ctx, c, func(ctx context.Context, t chainTarget) (llm.Stream, error) {
		return t.model.Stream(ctx, req)
	})
}

// chainDo runs the head-to-tail failover algorithm around an attempt
// function, generic over the result type (response vs stream).
func chainDo[T any](ctx context.Context, c *chain, attempt func(context.Context, chainTarget) (T, error)) (T, error) {
	var zero T
	var failures []error

	for _, t := range c.targets {
		if !c.tracker.Available(t.key) {
			until := c.tracker.BackedOffUntil(t.key)
			failures = append(failures, fmt.Errorf("%s: skipped (backed off until %s)", t.key, until.Format("15:04:05.000")))
			continue
		}

		retries := c.cfg.retries()
		for attemptN := 0; ; attemptN++ {
			if err := ctx.Err(); err != nil {
				return zero, err
			}
			result, err := attempt(ctx, t)
			if err == nil {
				c.tracker.ReportSuccess(t.key)
				return result, nil
			}

			class := c.cfg.classify(err)
			if class == llm.ClassPermanent {
				if errors.Is(err, llm.ErrModelNotFound) || c.cfg.AdvanceOnPermanent {
					// Not a health problem (or policy says keep going):
					// advance without penalizing the target.
					failures = append(failures, fmt.Errorf("%s: %w", t.key, err))
					break
				}
				// Failing over cannot fix a bad request or bad credentials.
				return zero, fmt.Errorf("%s: %w", t.key, err)
			}

			// Transient: every failed attempt counts toward the target's
			// consecutive-failure threshold. Retry the same target while
			// attempts remain — but advance as soon as the tracker benches
			// it (a freshly backed-off target is not worth more retries).
			benched := c.tracker.ReportFailure(t.key)
			if !benched && attemptN < retries {
				continue
			}
			failures = append(failures, fmt.Errorf("%s: %w", t.key, err))
			break
		}
	}

	return zero, errors.Join(append([]error{ErrChainExhausted}, failures...)...)
}