package billcost

import "fmt"

// CalculateShipCost computes the cost of shipping a parcel weighing
// weightG grams using the given province's bands. The bands must
// already be filtered to a single partition (the function does not
// inspect Band.Partition); callers that hold a multi-partition slice
// should group by Partition first.
//
// Algorithm:
//  1. Find the band b where b.LimitBottom < weightG <= b.LimitTop.
//  2. cost := b.BaseAmount (first-weight flat charge).
//  3. If weightG > b.BaseWeight and b.IncrementWeight > 0:
//     extra := weightG - b.BaseWeight
//     units := ceil(extra / b.IncrementWeight)
//     cost += units * b.IncrementPrice
//
// Returns an "uncovered" error when no band covers weightG, which
// the reflector treats as a coverage / gap rule violation.
//
// CalculateShipCost 给出按 bands 算的运费. bands 必须已过滤到单一
// 分区 (函数不检查 Band.Partition); 持多分区切片的调用方先按 Partition
// group.
//
// 算法:
//  1. 找命中段 b: b.LimitBottom < weightG <= b.LimitTop.
//  2. cost := b.BaseAmount (首重一口价).
//  3. weightG > b.BaseWeight 且 b.IncrementWeight > 0 时:
//     extra := weightG - b.BaseWeight
//     units := ceil(extra / b.IncrementWeight)
//     cost += units * b.IncrementPrice
//
// 无段命中返 "uncovered" 错, 反射器记 coverage / gap 类违规.
func CalculateShipCost(bands []Band, weightG int) (float64, error) {
	for _, b := range bands {
		if weightG > b.LimitBottom && weightG <= b.LimitTop {
			cost := b.BaseAmount
			if weightG > b.BaseWeight && b.IncrementWeight > 0 {
				extra := weightG - b.BaseWeight
				units := ceilDiv(extra, b.IncrementWeight)
				cost += float64(units) * b.IncrementPrice
			}
			return cost, nil
		}
	}
	return 0, fmt.Errorf("billcost: weight %dg uncovered by bands", weightG)
}

// ceilDiv returns ceil(a/b) for non-negative a and positive b.
//
// ceilDiv 返 ceil(a/b), 要求 a 非负 b 正.
func ceilDiv(a, b int) int {
	if b <= 0 {
		return 0
	}
	if a <= 0 {
		return 0
	}
	return (a + b - 1) / b
}