3.1 Three in One: fixed stack implementation

.gitignore

@@ -0,0 +1,3 @@
+# IDE configs
+.idea
+.vscode

README.md

@@ -36,7 +36,7 @@ Solving programming questions from ["Cracking the Coding Interview 6th Edition"]
 
 | # | Problem                       | Tested        | Solved    |
 |---|-------------------------------|:-------------:|:---------:|
-|3.1| Three in one                  | tests         |           |
+|3.1| [Three in one][39]            | [tests][40]   |     ✓     |
 |3.2| Stack Min                     | tests         |           |
 |3.3| Stack of Plates               | tests         |           |
 |3.4| Queue via Stacks              | tests         |           |
@@ -81,3 +81,5 @@ Solving programming questions from ["Cracking the Coding Interview 6th Edition"]
 [36]:  ch03/stack_test.go
 [37]:  ch03/queue.go
 [38]:  ch03/queue_test.go
+[39]:  ch03/01_three_in_one.go
+[49]:  ch03/01_three_in_one_test.go

ch03/01_three_in_one.go

@@ -0,0 +1,86 @@
+package ch03
+
+import "errors"
+
+// FixedMultiStack structure implements multiple stacks
+// on top of the flat slice
+type FixedMultiStack struct {
+	n       int   // amount of stacks in MultiStack
+	cap     int   // capacity of each n-th stack
+	len     []int // length of each n-th stack
+	storage []int // the main storage of stacks, flat slice
+}
+
+// NewFixedMultiStack returns empty initialized MultiStack
+func NewFixedMultiStack(n int, cap int) (*FixedMultiStack, error) {
+	if n < 1 {
+		return nil, errors.New("number of stacks is less than 1")
+	}
+
+	if cap < 1 {
+		return nil, errors.New("stack capacity is less than 1")
+	}
+
+	stack := new(FixedMultiStack)
+
+	stack.n = n
+	stack.cap = cap
+
+	stack.len = make([]int, n)
+	stack.storage = make([]int, n*cap)
+
+	return stack, nil
+}
+
+// Pop returns (and removes) the top item from the n-th stack
+func (stack *FixedMultiStack) Pop(n int) (int, error) {
+	if stack.IsEmpty(n) {
+		return 0, errors.New("empty stack error")
+	}
+
+	top := stack.top(n)
+	val := stack.storage[top]
+	stack.len[n]--
+
+	return val, nil
+}
+
+// Push add an item to the n-th stack
+func (stack *FixedMultiStack) Push(n int, val int) error {
+	if stack.IsFull(n) {
+		return errors.New("full stack error")
+	}
+
+	top := stack.top(n)
+	stack.storage[top] = val
+	stack.len[n]++
+
+	return nil
+}
+
+// Peek returns the top item from the n-th stack (without removing it)
+func (stack *FixedMultiStack) Peek(n int) (int, error) {
+	if stack.IsEmpty(n) {
+		return 0, errors.New("empty stack error")
+	}
+
+	top := stack.top(n)
+	return stack.storage[top], nil
+}
+
+// IsEmpty returns true when the n-th stack is empty, false otherwise
+func (stack *FixedMultiStack) IsEmpty(n int) bool {
+	return stack.len[n] == 0
+}
+
+// IsFull returns true when the n-th stack is full, false otherwise
+func (stack *FixedMultiStack) IsFull(n int) bool {
+	return stack.len[n] == stack.cap
+}
+
+// Top returns top element index for n-th stack
+// Top element's index calculated as:
+// n-th stack offset + top index in n-th stack
+func (stack *FixedMultiStack) top(n int) int {
+	return n*stack.cap + stack.len[n] - 1
+}