// Package budget tracks agent spend against rolling per-provider windows so the
// dispatcher can refuse (or reroute) paid work that would breach a cap. Local
// runners are free and simply carry no configured limit.
package budget

import (
	"sort"
	"time"
)

// DefaultWindow is the rolling spend window when none is configured.
const DefaultWindow = 5 * time.Hour

// Limits configures per-provider spend caps within a rolling window.
type Limits struct {
	// Window is the rolling lookback; DefaultWindow when zero.
	Window time.Duration
	// PerProvider maps a provider (claude/gemini/…) to its USD cap within the
	// window. A missing or non-positive entry means "unlimited" (e.g. local).
	PerProvider map[string]float64
}

// SpendSource supplies total spend per provider since a given time.
type SpendSource interface {
	SpendByProviderSince(since time.Time) (map[string]float64, error)
}

// Headroom describes remaining budget for one provider in the current window.
type Headroom struct {
	Provider  string  `json:"provider"`
	Limited   bool    `json:"limited"`
	Limit     float64 `json:"limit_usd"`
	Spent     float64 `json:"spent_usd"`
	Remaining float64 `json:"remaining_usd"`
	// Fraction is the share of the cap still available, 0..1 (0 when unlimited).
	Fraction float64 `json:"fraction_remaining"`
}

// Accountant answers spend questions against a SpendSource and Limits.
type Accountant struct {
	src SpendSource
	lim Limits
	now func() time.Time
}

// New builds an Accountant. A zero Window defaults to DefaultWindow.
func New(src SpendSource, lim Limits) *Accountant {
	if lim.Window <= 0 {
		lim.Window = DefaultWindow
	}
	return &Accountant{src: src, lim: lim, now: time.Now}
}

func (a *Accountant) since() time.Time { return a.now().Add(-a.lim.Window) }

func (a *Accountant) headroomFrom(provider string, spends map[string]float64) Headroom {
	limit := a.lim.PerProvider[provider]
	if limit <= 0 {
		return Headroom{Provider: provider, Limited: false}
	}
	spent := spends[provider]
	remaining := limit - spent
	if remaining < 0 {
		remaining = 0
	}
	return Headroom{
		Provider:  provider,
		Limited:   true,
		Limit:     limit,
		Spent:     spent,
		Remaining: remaining,
		Fraction:  remaining / limit,
	}
}

// Headroom returns the remaining budget for one provider in the current window.
func (a *Accountant) Headroom(provider string) (Headroom, error) {
	if a.lim.PerProvider[provider] <= 0 {
		return Headroom{Provider: provider, Limited: false}, nil
	}
	spends, err := a.src.SpendByProviderSince(a.since())
	if err != nil {
		return Headroom{}, err
	}
	return a.headroomFrom(provider, spends), nil
}

// Allow reports whether starting a task on provider that may cost up to estCost
// is permitted without breaching the window cap. Unlimited providers always
// pass. estCost is typically the task's max_budget_usd (0 = unknown).
func (a *Accountant) Allow(provider string, estCost float64) (bool, error) {
	h, err := a.Headroom(provider)
	if err != nil {
		return false, err
	}
	if !h.Limited {
		return true, nil
	}
	return h.Spent+estCost <= h.Limit, nil
}

// All returns headroom for every configured provider, sorted by name, in a
// single spend query.
func (a *Accountant) All() ([]Headroom, error) {
	if len(a.lim.PerProvider) == 0 {
		return []Headroom{}, nil
	}
	spends, err := a.src.SpendByProviderSince(a.since())
	if err != nil {
		return nil, err
	}
	providers := make([]string, 0, len(a.lim.PerProvider))
	for p := range a.lim.PerProvider {
		providers = append(providers, p)
	}
	sort.Strings(providers)
	out := make([]Headroom, 0, len(providers))
	for _, p := range providers {
		out = append(out, a.headroomFrom(p, spends))
	}
	return out, nil
}