package billcost

import (
	"testing"
)

// TestCalculateShipCost_FlatBand verifies the flat-fee case
// (weight <= BaseWeight pays BaseAmount with no continuation).
//
// TestCalculateShipCost_FlatBand 验一口价段 (weight <= BaseWeight
// 按 BaseAmount, 不计续重).
func TestCalculateShipCost_FlatBand(t *testing.T) {
	bands := []Band{{
		LimitBottom: 0, LimitTop: 1000,
		BaseWeight: 1000, BaseAmount: 6.0,
	}}
	cases := []struct {
		w    int
		want float64
	}{
		{1, 6.0},
		{500, 6.0},
		{1000, 6.0},
	}
	for _, c := range cases {
		got, err := CalculateShipCost(bands, c.w)
		if err != nil {
			t.Errorf("weight %d: unexpected error %v", c.w, err)
			continue
		}
		if got != c.want {
			t.Errorf("weight %d: got %.2f, want %.2f", c.w, got, c.want)
		}
	}
}

// TestCalculateShipCost_Continuation verifies the
// base-weight + continuation calculation, including ceil-rounding
// of partial increment units.
//
// TestCalculateShipCost_Continuation 验首重 + 续重 (含部分增量
// ceil 取整).
func TestCalculateShipCost_Continuation(t *testing.T) {
	// 1kg first weight = 6元, +3元 per 1kg continuation.
	bands := []Band{{
		LimitBottom: 0, LimitTop: 50000,
		BaseWeight: 1000, BaseAmount: 6.0,
		IncrementWeight: 1000, IncrementPrice: 3.0,
	}}
	cases := []struct {
		w    int
		want float64
	}{
		{1000, 6.0},     // exactly base
		{1001, 9.0},     // 1g over -> ceil(1/1000) = 1 unit -> +3
		{2000, 9.0},     // 1kg continuation
		{2001, 12.0},    // 1.001kg continuation -> ceil(1001/1000) = 2 -> +6
		{3000, 12.0},    // 2kg continuation
		{49000, 150.0},  // 48kg continuation -> +144 -> 150
	}
	for _, c := range cases {
		got, err := CalculateShipCost(bands, c.w)
		if err != nil {
			t.Errorf("weight %d: unexpected error %v", c.w, err)
			continue
		}
		if got != c.want {
			t.Errorf("weight %d: got %.2f, want %.2f", c.w, got, c.want)
		}
	}
}

// TestCalculateShipCost_Uncovered ensures weights outside any band
// raise an error rather than returning zero (silent miss).
//
// TestCalculateShipCost_Uncovered 验段外重量返错而非静默 0.
func TestCalculateShipCost_Uncovered(t *testing.T) {
	bands := []Band{{
		LimitBottom: 0, LimitTop: 1000,
		BaseWeight: 1000, BaseAmount: 6.0,
	}}
	if _, err := CalculateShipCost(bands, 1001); err == nil {
		t.Error("weight beyond LimitTop: want error, got nil")
	}
	if _, err := CalculateShipCost(bands, 0); err == nil {
		t.Error("weight 0 (boundary open lower): want error, got nil")
	}
}

// TestCalculateShipCost_MultiBand verifies multi-segment lookup
// hits the correct band.
//
// TestCalculateShipCost_MultiBand 验多段表的命中段查找.
func TestCalculateShipCost_MultiBand(t *testing.T) {
	bands := []Band{
		{LimitBottom: 0, LimitTop: 1000, BaseWeight: 1000, BaseAmount: 5.0},
		{LimitBottom: 1000, LimitTop: 3000, BaseWeight: 1000, BaseAmount: 5.0,
			IncrementWeight: 1000, IncrementPrice: 2.0},
		{LimitBottom: 3000, LimitTop: 50000, BaseWeight: 3000, BaseAmount: 9.0,
			IncrementWeight: 1000, IncrementPrice: 1.5},
	}
	cases := []struct {
		w    int
		want float64
	}{
		{500, 5.0},     // band 1 flat
		{1500, 7.0},    // band 2: 5 + ceil(500/1000)*2 = 5+2 = 7
		{3000, 9.0},    // band 2 last weight: 5 + ceil(2000/1000)*2 = 5+4 = 9
		{3001, 10.5},   // band 3: 9 + ceil(1/1000)*1.5 = 10.5
		{4000, 10.5},   // band 3: 9 + ceil(1000/1000)*1.5 = 10.5
		{49000, 78.0},  // band 3: 9 + 46*1.5 = 78
	}
	for _, c := range cases {
		got, err := CalculateShipCost(bands, c.w)
		if err != nil {
			t.Errorf("weight %d: unexpected error %v", c.w, err)
			continue
		}
		if got != c.want {
			t.Errorf("weight %d: got %.2f, want %.2f", c.w, got, c.want)
		}
	}
}

// TestCeilDiv covers the small helper.
//
// TestCeilDiv 小辅助函数.
func TestCeilDiv(t *testing.T) {
	cases := []struct {
		a, b, want int
	}{
		{0, 1000, 0},
		{1, 1000, 1},
		{999, 1000, 1},
		{1000, 1000, 1},
		{1001, 1000, 2},
		{2000, 1000, 2},
		{2001, 1000, 3},
		{-1, 1000, 0},
		{1000, 0, 0},
	}
	for _, c := range cases {
		if got := ceilDiv(c.a, c.b); got != c.want {
			t.Errorf("ceilDiv(%d, %d) = %d, want %d", c.a, c.b, got, c.want)
		}
	}
}