2 files changed, 134 insertions, 0 deletions
diff --git a/engine/pool/pool.go b/engine/pool/pool.go
new file mode 100644
index 0000000..2179a12
--- /dev/null
+++ b/engine/pool/pool.go
@@ -0,0 +1,58 @@
+package pool
+
+const Infinite = -1
+const Empty = 0
+
+type Pool struct {
+	OverflowsTo *Pool
+	SubtractsTo *Pool
+	Max         int
+	Value       int
+}
+
+func (p *Pool) Add(amount int) {
+	p.Value += amount
+	if p.Max == Infinite {
+		return
+	}
+
+	if p.Value > p.Max {
+		overflow := p.Value - p.Max
+		p.Value = p.Max
+		p.OverflowsTo.Add(overflow)
+	}
+}
+
+func (p *Pool) Subtract(amount int) {
+	if p.SubtractsTo != nil {
+		forward := amount
+		if forward > p.Value {
+			forward = p.Value
+		}
+		p.SubtractsTo.Add(forward)
+	}
+
+	if p.Value == Infinite {
+		return
+	}
+
+	p.Value -= amount
+	if p.Value < 0 {
+		p.Value = 0
+	}
+}
+
+func (p *Pool) Fits(amount int) bool {
+	return p.Value+amount <= p.Max
+}
+
+func (p *Pool) Has(amount int) bool {
+	return p.Value >= amount
+}
+
+func New(start, max int) *Pool {
+	return &Pool{
+		Max:   max,
+		Value: start,
+	}
+}
diff --git a/engine/pool/pool_test.go b/engine/pool/pool_test.go
new file mode 100644
index 0000000..4dd0ea5
--- /dev/null
+++ b/engine/pool/pool_test.go
@@ -0,0 +1,76 @@
+package pool
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestPool_Add(t *testing.T) {
+	p := &Pool{
+		Max:   10,
+		Value: 3,
+	}
+	p.Add(4)
+}
+
+func TestPool_Add_Overflows(t *testing.T) {
+	shad := &Pool{
+		Max:   5,
+		Value: 0,
+	}
+	dist := &Pool{
+		OverflowsTo: shad,
+		Max:         5,
+		Value:       0,
+	}
+	dist.Add(8)
+	assert.Equal(t, shad.Value, 3)
+}
+
+func TestPool_Subtract(t *testing.T) {
+	p := &Pool{
+		Max:   10,
+		Value: 8,
+	}
+	p.Subtract(6)
+	assert.Equal(t, 2, p.Value)
+}
+
+func TestPool_Subtract_Empties(t *testing.T) {
+	p := &Pool{
+		Max:   10,
+		Value: 4,
+	}
+	p.Subtract(8)
+	assert.Equal(t, 0, p.Value)
+}
+
+func TestPool_Subtract_Flows(t *testing.T) {
+	dest := &Pool{
+		Value: 0,
+		Max:   Infinite,
+	}
+	p := &Pool{
+		SubtractsTo: dest,
+		Max:         Infinite,
+		Value:       10,
+	}
+	p.Subtract(8)
+	assert.Equal(t, 8, dest.Value)
+	assert.Equal(t, 2, p.Value)
+}
+
+func TestPool_Subtract_Flows_Empties(t *testing.T) {
+	dest := &Pool{
+		Value: 0,
+		Max:   Infinite,
+	}
+	p := &Pool{
+		SubtractsTo: dest,
+		Max:         Infinite,
+		Value:       5,
+	}
+	p.Subtract(8)
+	assert.Equal(t, 5, dest.Value)
+	assert.Equal(t, 0, p.Value)
+}