new-api-custom/pkg/billingexpr/detection_bound.go

package billingexpr

import (
	"errors"
	"math"

	"github.com/expr-lang/expr/ast"
	"github.com/expr-lang/expr/parser"
)

// DetectionCostBound proves a conservative cost ceiling rather than sampling
// a few token counts. Both conditional branches are bounded, including prices
// selected by time/request rules. Unsupported expressions fail closed.
func DetectionCostBound(expression string, input, output float64) (float64, error) {
	_, body := ParseExprVersion(expression)
	tree, err := parser.Parse(body)
	if err != nil {
		return 0, err
	}
	var bound func(ast.Node) (float64, error)
	unsupported := errors.New("detection pricing cannot be safely bounded")
	bound = func(node ast.Node) (float64, error) {
		switch n := node.(type) {
		case *ast.IntegerNode:
			if n.Value >= 0 {
				return float64(n.Value), nil
			}
		case *ast.FloatNode:
			if n.Value >= 0 {
				return n.Value, nil
			}
		case *ast.IdentifierNode:
			switch n.Value {
			case "p", "len", "cr", "cc", "cc1h":
				return input, nil
			case "c":
				return output, nil
			case "img", "img_cr", "img_o", "ai", "ao":
				return 0, nil
			case "image_count":
				return 1, nil
			}
		case *ast.BinaryNode:
			a, e := bound(n.Left)
			if e != nil {
				return 0, e
			}
			b, e := bound(n.Right)
			if e != nil {
				return 0, e
			}
			switch n.Operator {
			case "+":
				return a + b, nil
			case "*":
				return a * b, nil
			case "/":
				switch n.Right.(type) {
				case *ast.IntegerNode, *ast.FloatNode:
					if b > 0 {
						return a / b, nil
					}
				}
			}
		case *ast.ConditionalNode:
			a, e := bound(n.Exp1)
			if e != nil {
				return 0, e
			}
			b, e := bound(n.Exp2)
			return max(a, b), e
		case *ast.CallNode:
			name, ok := n.Callee.(*ast.IdentifierNode)
			if !ok {
				return 0, unsupported
			}
			if name.Value == "tier" && len(n.Arguments) == 2 {
				return bound(n.Arguments[1])
			}
			if name.Value == "fixed" && len(n.Arguments) == 1 {
				v, e := bound(n.Arguments[0])
				return v * 1_000_000, e
			}
			if name.Value == "ceil" && len(n.Arguments) == 1 {
				v, e := bound(n.Arguments[0])
				return math.Ceil(v), e
			}
			if name.Value == "floor" && len(n.Arguments) == 1 {
				v, e := bound(n.Arguments[0])
				return math.Floor(v), e
			}
			if name.Value == "min" || name.Value == "max" {
				v := float64(0)
				for _, arg := range n.Arguments {
					x, e := bound(arg)
					if e != nil {
						return 0, e
					}
					v = max(v, x)
				}
				return v, nil
			}
		}
		return 0, unsupported
	}
	value, err := bound(tree.Node)
	if math.IsNaN(value) || math.IsInf(value, 0) {
		return 0, unsupported
	}
	return value, err
}