ModelRouter/internal/billing/compile.go

package billing

import (
	"fmt"
	"strings"

	"llmsproxy/internal/config"
)

// Compile turns one billing profile into the prices table the billing plugin
// expects, resolving URL rules against the gateway's actual sources.
//
// WHY URL RULES NEED RESOLVING AT ALL: the operator declares pricing by URL
// because that is what a provider's price list is keyed on, and because several
// sources can point at the same URL. The plugin, however, looks up by
// `payload.source` (a source NAME) and by model — it has no idea what URL a
// request went to. So the URL match happens here, at compile time, where the
// config's name->base_url mapping is known, and the result is expressed in the
// dimensions the plugin already supports.
//
// Profiles are the "let the user choose" axis: the same URL can appear in
// several profiles and switching recomputes this table, so a gateway can be
// repriced without editing the plugin.
func Compile(profile *config.BillingProfile, sources []config.Source) (map[string]interface{}, error) {
	if profile == nil {
		return nil, fmt.Errorf("no billing profile")
	}
	prices := map[string]interface{}{
		"currency": "USD",
		"default":  map[string]interface{}{"prompt": 0.0, "completion": 0.0, "per_request": 0.0},
		"sources":  map[string]interface{}{},
		"models":   map[string]interface{}{},
		"keys":     map[string]interface{}{},
	}
	if profile.Default != "" {
		prices["default_mode"] = profile.Default
	}

	// Currency is a profile-level statement; a rule may override it.
	for i := range profile.Rules {
		if c := profile.Rules[i].Currency; c != "" {
			prices["currency"] = c
			break
		}
	}

	matched := map[string]bool{}
	for i := range profile.Rules {
		rule := &profile.Rules[i]
		targets := matchSources(rule.URL, sources)
		if len(targets) == 0 && rule.URL != "*" {
			// A rule for a URL no source uses is almost always a typo (or a
			// source that was removed). Failing loudly beats a profile that
			// silently prices nothing — the whole reason this is a config file
			// instead of a hand-written JSON blob.
			return nil, fmt.Errorf("rule url %q matches no configured source base_url", rule.URL)
		}
		for _, srcName := range targets {
			if matched[srcName] {
				// First rule wins. Two rules matching one source is ambiguous,
				// and silently letting the later one win makes the file's
				// meaning depend on ordering the operator cannot see.
				continue
			}
			matched[srcName] = true
			if err := applyRule(prices, rule, srcName, sources); err != nil {
				return nil, err
			}
		}
	}

	// Sources no rule matched fall through to the profile default, which the
	// plugin applies via prices.default. Recording them explicitly means the
	// UI can say "this source is unpriced" instead of leaving the operator to
	// infer it from a zero.
	var unmatched []string
	for _, src := range sources {
		if !matched[src.Name] {
			unmatched = append(unmatched, src.Name)
		}
	}
	if len(unmatched) > 0 {
		prices["unmatched_sources"] = unmatched
	}
	return prices, nil
}

// matchSources returns the source names whose base_url matches pattern.
// "*" matches every source (used as a catch-all default rule).
func matchSources(pattern string, sources []config.Source) []string {
	var out []string
	if pattern == "*" {
		for _, s := range sources {
			out = append(out, s.Name)
		}
		return out
	}
	want := normalizeURL(pattern)
	for _, s := range sources {
		if normalizeURL(s.BaseURL) == want {
			out = append(out, s.Name)
		}
	}
	return out
}

// normalizeURL compares URLs the way an operator expects: trailing slashes and
// case in the host are differences the provider's price list does not care
// about, and requiring an exact byte match would make the config brittle.
func normalizeURL(u string) string {
	u = strings.TrimSpace(u)
	u = strings.TrimRight(u, "/")
	return strings.ToLower(u)
}

// applyRule writes one rule's pricing for one source into the prices table.
func applyRule(prices map[string]interface{}, rule *config.BillingRule, srcName string, sources []config.Source) error {
	switch rule.Mode {
	case "free":
		// Explicitly priced at zero. This is NOT the same as unpriced: a source
		// the operator says is free must not appear in the unpriced warnings,
		// or those warnings become noise and stop being read.
		prices["sources"].(map[string]interface{})[srcName] = map[string]interface{}{
			"prompt": 0.0, "completion": 0.0, "per_request": 0.0,
		}
		return nil

	case "subscription":
		// A fixed monthly commitment: the per-request MARGINAL cost is zero, and
		// the flat fee is reported separately. Spreading a monthly fee across
		// requests would invent a per-request number the provider never charges,
		// and it would change every time traffic did.
		prices["sources"].(map[string]interface{})[srcName] = map[string]interface{}{
			"prompt": 0.0, "completion": 0.0, "per_request": 0.0,
		}
		fixed, _ := prices["subscriptions"].(map[string]interface{})
		if fixed == nil {
			fixed = map[string]interface{}{}
			prices["subscriptions"] = fixed
		}
		cur := "USD"
		if rule.Currency != "" {
			cur = rule.Currency
		}
		fixed[srcName] = map[string]interface{}{
			"monthly": rule.Subscription, "currency": cur,
		}
		return nil

	case "unpriced":
		// Deliberately left out of `sources` so the plugin's unpriced_models /
		// unpriced_reqs counters catch it. That is the point: a subscription
		// plan whose credits cannot be converted to tokens must be VISIBLE as
		// unbilled, not quietly estimated.
		return nil

	case "token":
		if len(rule.Models) == 0 {
			return fmt.Errorf("rule for url %q: mode token requires at least one model", rule.URL)
		}
		models := prices["models"].(map[string]interface{})
		for modelID, t := range rule.Models {
			entry := map[string]interface{}{}
			prompt, err := usdPerM(t.Prompt)
			if err != nil {
				return fmt.Errorf("model %q prompt: %w", modelID, err)
			}
			completion, err := usdPerM(t.Completion)
			if err != nil {
				return fmt.Errorf("model %q completion: %w", modelID, err)
			}
			entry["prompt"] = prompt
			entry["completion"] = completion
			if t.CacheDiscount != nil {
				entry["cache_discount"] = *t.CacheDiscount
			}
			if rule.Peak != nil {
				entry["peak"] = compilePeak(rule.Peak)
			}
			models[modelID] = entry
		}
		// The source itself needs an entry too, otherwise priceFor() marks the
		// request priced only when the model happens to be listed, and a model
		// served from this URL but absent from the rule would be billed at the
		// DEFAULT (zero) rate — indistinguishable from unpriced in the totals.
		if _, ok := prices["sources"].(map[string]interface{})[srcName]; !ok {
			prices["sources"].(map[string]interface{})[srcName] = map[string]interface{}{
				"prompt": 0.0, "completion": 0.0, "per_request": 0.0,
			}
		}
		return nil
	}
	return fmt.Errorf("unknown mode %q", rule.Mode)
}

// usdPerM converts a USD-per-million string into the per-token rate the plugin
// expects. Delegates to config.ParseFloatUSDPerM so the DSL validator and the
// compiler agree byte-for-byte on what a valid price is — a value that passes
// Validate() but fails here (or vice versa) would be the worst kind of drift.
func usdPerM(s string) (float64, error) {
	return config.ParseFloatUSDPerM(s)
}

// compilePeak renders the peak window in the shape the plugin reads:
// { multiplier, windows = { { days = {...}, hours = { {lo,hi}, ... } } } }.
//
// The plugin reads `days` / `hours` pairs. The first version of the deployed
// price table used { start, end, weekdays } — a shape nothing reads — so peak
// traffic was billed at off-peak rates with no error anywhere. Compiling from
// typed config fields removes the chance of writing the wrong key names by hand.
func compilePeak(p *config.BillingPeak) map[string]interface{} {
	win := map[string]interface{}{}
	if len(p.Weekdays) > 0 {
		days := make([]interface{}, 0, len(p.Weekdays))
		for _, d := range p.Weekdays {
			days = append(days, d)
		}
		win["days"] = days
	}
	if len(p.Hours) > 0 {
		hours := make([]interface{}, 0, len(p.Hours))
		for _, h := range p.Hours {
			hours = append(hours, []interface{}{h[0], h[1]})
		}
		win["hours"] = hours
	}
	out := map[string]interface{}{"multiplier": p.Multiplier}
	if len(win) > 0 {
		out["windows"] = []interface{}{win}
	}
	return out
}