package decimal

import (
	"math"
	"testing"
)

const epsilon = 0.001

func almostEqual(a, b float64) bool { return math.Abs(a-b) < epsilon }

func TestPricePerLiter(t *testing.T) {
	cases := []struct {
		name           string
		total, liters  float64
		expect         float64
	}{
		{"plan example 176.33/23.67", 176.33, 23.67, 7.45},
		{"simple 320/40", 320, 40, 8.0},
		{"simple 256/32", 256, 32, 8.0},
		{"zero liters", 100, 0, 0},
		{"negative liters", 100, -1, 0},
		{"precision 23.67/3.18", 23.67, 3.18, 7.4434}, // 23.67/3.18 = 7.4433...
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			got := PricePerLiter(c.total, c.liters)
			if !almostEqual(got, c.expect) {
				t.Errorf("PricePerLiter(%v,%v) = %v, want %v", c.total, c.liters, got, c.expect)
			}
		})
	}
}

func TestConsumptionL100km(t *testing.T) {
	cases := []struct {
		name           string
		liters, mileage float64
		expect         float64
	}{
		{"plan example (40/500*100)", 40, 500, 8.0},
		{"32/500*100", 32, 500, 6.4},
		{"zero mileage", 40, 0, 0},
		{"negative mileage", 40, -1, 0},
		{"rounding 2dp", 7.123, 100, 7.12}, // 7.123/100*100 = 7.123 -> round 2dp
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			got := ConsumptionL100km(c.liters, c.mileage)
			if !almostEqual(got, c.expect) {
				t.Errorf("ConsumptionL100km(%v,%v) = %v, want %v", c.liters, c.mileage, got, c.expect)
			}
		})
	}
}

func TestCostPerKm(t *testing.T) {
	cases := []struct {
		name           string
		amount, mileage float64
		expect         float64
	}{
		{"plan example 256/500", 256, 500, 0.512},
		{"zero mileage", 100, 0, 0},
		{"negative mileage", 100, -1, 0},
		{"typical 200/400", 200, 400, 0.5},
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			got := CostPerKm(c.amount, c.mileage)
			if !almostEqual(got, c.expect) {
				t.Errorf("CostPerKm(%v,%v) = %v, want %v", c.amount, c.mileage, got, c.expect)
			}
		})
	}
}

func TestMileage(t *testing.T) {
	cases := []struct {
		name           string
		current, prev  float64
		expect         float64
	}{
		{"normal increase", 11000, 10500, 500},
		{"equal (no travel)", 10500, 10500, 0},
		{"odometer rollback (typo)", 10400, 10500, 0},
		{"zero prev (first record)", 10000, 0, 10000},
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			got := Mileage(c.current, c.prev)
			if !almostEqual(got, c.expect) {
				t.Errorf("Mileage(%v,%v) = %v, want %v", c.current, c.prev, got, c.expect)
			}
		})
	}
}

func TestAvgFloat64(t *testing.T) {
	cases := []struct {
		name   string
		xs     []float64
		expect float64
	}{
		{"empty", nil, 0},
		{"single", []float64{8.0}, 8.0},
		{"two", []float64{7.0, 9.0}, 8.0},
		{"three typical", []float64{6.0, 7.0, 8.0}, 7.0},
		{"rounding", []float64{0.123, 0.456, 0.789}, 0.46}, // avg = 0.456
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			got := AvgFloat64(c.xs)
			if !almostEqual(got, c.expect) {
				t.Errorf("AvgFloat64(%v) = %v, want %v", c.xs, got, c.expect)
			}
		})
	}
}