feat: implement tiered billing expression evaluation and related functionality

- Added support for tiered billing expressions in the billing system.
- Introduced new types and functions for handling billing expressions, including caching and execution.
- Updated existing billing logic to accommodate tiered billing scenarios.
- Enhanced request handling to support incoming billing expression requests.
- Added tests for tiered billing functionality to ensure correctness.

pkg/billingexpr/run.go

@@ -0,0 +1,139 @@
+package billingexpr
+
+import (
+	"fmt"
+	"math"
+	"strings"
+	"time"
+
+	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/vm"
+	"github.com/tidwall/gjson"
+)
+
+// RunExpr compiles (with cache) and executes an expression string.
+// The environment exposes:
+//   - p, c             — prompt / completion tokens
+//   - cr, cc, cc1h     — cache read / creation / creation-1h tokens
+//   - tier(name, value) — trace callback that records which tier matched
+//   - max, min, abs, ceil, floor — standard math helpers
+//
+// Returns the resulting float64 quota (before group ratio) and a TraceResult
+// with side-channel info captured by tier() during execution.
+func RunExpr(exprStr string, params TokenParams) (float64, TraceResult, error) {
+	return RunExprWithRequest(exprStr, params, RequestInput{})
+}
+
+func RunExprWithRequest(exprStr string, params TokenParams, request RequestInput) (float64, TraceResult, error) {
+	prog, err := CompileFromCache(exprStr)
+	if err != nil {
+		return 0, TraceResult{}, err
+	}
+	return runProgram(prog, params, request)
+}
+
+// RunExprByHash is like RunExpr but accepts a pre-computed hash for the cache
+// lookup, avoiding a redundant SHA-256 computation when the caller already
+// holds BillingSnapshot.ExprHash.
+func RunExprByHash(exprStr, hash string, params TokenParams) (float64, TraceResult, error) {
+	return RunExprByHashWithRequest(exprStr, hash, params, RequestInput{})
+}
+
+func RunExprByHashWithRequest(exprStr, hash string, params TokenParams, request RequestInput) (float64, TraceResult, error) {
+	prog, err := CompileFromCacheByHash(exprStr, hash)
+	if err != nil {
+		return 0, TraceResult{}, err
+	}
+	return runProgram(prog, params, request)
+}
+
+func runProgram(prog *vm.Program, params TokenParams, request RequestInput) (float64, TraceResult, error) {
+	trace := TraceResult{}
+	headers := normalizeHeaders(request.Headers)
+
+	env := map[string]interface{}{
+		"p":                      params.P,
+		"c":                      params.C,
+		"cr":                     params.CR,
+		"cc":                     params.CC,
+		"cc1h":                   params.CC1h,
+		"prompt_tokens":          params.P,
+		"completion_tokens":      params.C,
+		"cache_read_tokens":      params.CR,
+		"cache_create_tokens":    params.CC,
+		"cache_create_1h_tokens": params.CC1h,
+		"tier": func(name string, value float64) float64 {
+			trace.MatchedTier = name
+			trace.Cost = value
+			return value
+		},
+		"header": func(key string) string {
+			return headers[strings.ToLower(strings.TrimSpace(key))]
+		},
+		"param": func(path string) interface{} {
+			path = strings.TrimSpace(path)
+			if path == "" || len(request.Body) == 0 {
+				return nil
+			}
+			result := gjson.GetBytes(request.Body, path)
+			if !result.Exists() {
+				return nil
+			}
+			return result.Value()
+		},
+		"has": func(source interface{}, substr string) bool {
+			if source == nil || substr == "" {
+				return false
+			}
+			return strings.Contains(fmt.Sprint(source), substr)
+		},
+		"hour":    func(tz string) int { return timeInZone(tz).Hour() },
+		"minute":  func(tz string) int { return timeInZone(tz).Minute() },
+		"weekday": func(tz string) int { return int(timeInZone(tz).Weekday()) },
+		"month":   func(tz string) int { return int(timeInZone(tz).Month()) },
+		"day":     func(tz string) int { return timeInZone(tz).Day() },
+		"max":     math.Max,
+		"min":   math.Min,
+		"abs":   math.Abs,
+		"ceil":  math.Ceil,
+		"floor": math.Floor,
+	}
+
+	out, err := expr.Run(prog, env)
+	if err != nil {
+		return 0, trace, fmt.Errorf("expr run error: %w", err)
+	}
+	f, ok := out.(float64)
+	if !ok {
+		return 0, trace, fmt.Errorf("expr result is %T, want float64", out)
+	}
+	return f, trace, nil
+}
+
+func timeInZone(tz string) time.Time {
+	tz = strings.TrimSpace(tz)
+	if tz == "" {
+		return time.Now().UTC()
+	}
+	loc, err := time.LoadLocation(tz)
+	if err != nil {
+		return time.Now().UTC()
+	}
+	return time.Now().In(loc)
+}
+
+func normalizeHeaders(headers map[string]string) map[string]string {
+	if len(headers) == 0 {
+		return map[string]string{}
+	}
+	normalized := make(map[string]string, len(headers))
+	for key, value := range headers {
+		k := strings.ToLower(strings.TrimSpace(key))
+		v := strings.TrimSpace(value)
+		if k == "" || v == "" {
+			continue
+		}
+		normalized[k] = v
+	}
+	return normalized
+}