// Sub creates a new amount by subtracting a2 from a1
// TODO : more explanation
func Sub(a1, a2 Amount) Amount {
out := a1.Copy()
out = valByValSub(out, a2.quantsWithout())
for _, q := range a2.QuantsWithByFactAsc() {
if old, ok := out.quants[q.ID()]; !ok {
out.quants[q.ID()] = quant.Quant{q.Unit, -q.Val}
} else {
if old.Val >= q.Val || q.Val > out.TotalWith() {
out.quants[q.ID()] = quant.Sub(old, q)
continue
}
current := out.QuantsWithByFactAsc()
needed := quant.TrimSliceOnTotal(current, q.Total())
sort.Sort(quant.ByFactDesc(needed))
missing := q.Total()
for i, n := range needed {
left := quant.SliceTotal(needed[i+1:])
if left == missing {
continue
}
tosub := missing / n.Fact
if left < missing {
tosub = (missing - left) / n.Fact
if tosub*n.Fact != missing-left {
tosub++
}
}
out.quants[n.ID()] = n.NewSub(tosub)
missing -= tosub * n.Fact
}
if missing > 0 {
valByValSub(out, map[string]quant.Quant{q.ID(): {q.Unit, missing}})
}
}
}
return out
}
// QuantsByFactDesc returns the quantities of am by descending factor
func (am Amount) QuantsByFactDesc() []quant.Quant {
out := am.Quants()
sort.Sort(quant.ByFactDesc(out))
return out
}