package ledger

import (
	"testing"

	"textmachine/backend/internal/llm"
)

// TestTheThinkingShareOfACompletionPricesNothing keeps the diagnostic off the money path.
//
// llm.Usage carries two thinking counts and only one is money: ReasoningTokens is added to the
// completion here, ReasoningInCompletion is a SUBSET of a completion already priced. On the measured
// DeepSeek shape thinking is ~60% of a call, so pricing the subset field would inflate every bill on
// the only provider the shipping configs call.
//
// The assertion is built so the money is UNAVOIDABLE rather than likely: the completion is large, the
// output price is non-zero, and the baseline cost is required to be positive — a formula change that
// zeroed the output term would otherwise make this pass by comparing nothing with nothing.
func TestTheThinkingShareOfACompletionPricesNothing(t *testing.T) {
	price := ModelPrice{InputPerM: 1.32, CachedPerM: 0.044, OutputPerM: 3.96}
	base := llm.Usage{PromptTokens: 6908, CachedTokens: 1152, CompletionTokens: 16000}

	want := CostUSD(price, base)
	if want <= 0 {
		t.Fatalf("the fixture must actually cost money, else this test compares two zeros: %v", want)
	}

	for _, share := range []int{0, 1, 8000, 16000} {
		n := share
		withThinking := base
		withThinking.ReasoningInCompletion = &n
		if got := CostUSD(price, withThinking); got != want {
			t.Fatalf("a thinking share of %d moved the bill %v -> %v; the subset field is a diagnostic and must price nothing", share, want, got)
		}
	}

	// The other field still IS money, so a pin that passed for both would be measuring nothing.
	onTop := base
	onTop.ReasoningTokens = 16000
	if got := CostUSD(price, onTop); got <= want {
		t.Fatalf("reasoning billed ON TOP must raise the bill above %v, got %v — the control that proves this test can see a price move at all", want, got)
	}
}