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Merge pull request #25 from bvwells/interpreter
Implement interpreter design pattern #11
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1 +1,117 @@ | ||
| package behavioral | ||
|
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| import ( | ||
| "strings" | ||
| ) | ||
|
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||
| // Expression represents an expression to evaluate. | ||
| type Expression interface { | ||
| Interpret(variables *map[string]Expression) int | ||
| } | ||
|
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| // Integer represents an integer number. | ||
| type Integer struct { | ||
| integer int | ||
| } | ||
|
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| // Interpret returns the integer representation of the number. | ||
| func (n *Integer) Interpret(variables *map[string]Expression) int { | ||
| return n.integer | ||
| } | ||
|
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| // Plus represents the addition operation. | ||
| type Plus struct { | ||
| leftOperand Expression | ||
| rightOperand Expression | ||
| } | ||
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| // Interpret interprets by adding the left and right variables. | ||
| func (p *Plus) Interpret(variables *map[string]Expression) int { | ||
| return p.leftOperand.Interpret(variables) + p.rightOperand.Interpret(variables) | ||
| } | ||
|
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| // Minus represents the substraction operation. | ||
| type Minus struct { | ||
| leftOperand Expression | ||
| rightOperand Expression | ||
| } | ||
|
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| // Interpret interprets by subtracting the right from left variables. | ||
| func (m *Minus) Interpret(variables *map[string]Expression) int { | ||
| return m.leftOperand.Interpret(variables) - m.rightOperand.Interpret(variables) | ||
| } | ||
|
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| // Variable represents a variable. | ||
| type Variable struct { | ||
| name string | ||
| } | ||
|
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| // Interpret looks up the variable value and returns it, if not found returns zero. | ||
| func (v *Variable) Interpret(variables *map[string]Expression) int { | ||
| value, found := (*variables)[v.name] | ||
| if found == false { | ||
| return 0 | ||
| } | ||
| return value.Interpret(variables) | ||
| } | ||
|
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| // Evaluator evaluates the expression. | ||
| type Evaluator struct { | ||
| syntaxTree Expression | ||
| } | ||
|
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| // NewEvaluator creates a new Evaluator. | ||
| func NewEvaluator(expression string) *Evaluator { | ||
| expressionStack := new(Stack) | ||
| for _, token := range strings.Split(expression, " ") { | ||
| if token == "+" { | ||
| right := expressionStack.Pop().(Expression) | ||
| left := expressionStack.Pop().(Expression) | ||
| subExpression := &Plus{left, right} | ||
| expressionStack.Push(subExpression) | ||
| } else if token == "-" { | ||
| right := expressionStack.Pop().(Expression) | ||
| left := expressionStack.Pop().(Expression) | ||
| subExpression := &Minus{left, right} | ||
| expressionStack.Push(subExpression) | ||
| } else { | ||
| expressionStack.Push(&Variable{token}) | ||
| } | ||
| } | ||
| syntaxTree := expressionStack.Pop().(Expression) | ||
| return &Evaluator{syntaxTree} | ||
| } | ||
|
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| // Interpret interprets the expression syntax tree. | ||
| func (e *Evaluator) Interpret(context *map[string]Expression) int { | ||
| return e.syntaxTree.Interpret(context) | ||
| } | ||
|
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| // Node represents a node in the stack. | ||
| type Node struct { | ||
| value interface{} | ||
| next *Node | ||
| } | ||
|
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| // Stack represents a stack with push and pop operations. | ||
| type Stack struct { | ||
| top *Node | ||
| size int | ||
| } | ||
|
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| // Push pushes a new value into the stack. | ||
| func (s *Stack) Push(value interface{}) { | ||
| s.top = &Node{value, s.top} | ||
| s.size++ | ||
| } | ||
|
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| // Pop pops a value out the stack. | ||
| func (s *Stack) Pop() interface{} { | ||
| if s.size == 0 { | ||
| return nil | ||
| } | ||
| value := s.top.value | ||
| s.top = s.top.next | ||
| s.size-- | ||
| return value | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,18 @@ | ||
| package behavioral | ||
|
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| import ( | ||
| "testing" | ||
|
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| "github.com/stretchr/testify/assert" | ||
| ) | ||
|
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| func TestInterpreter(t *testing.T) { | ||
| expression := "w x z - +" | ||
| sentence := NewEvaluator(expression) | ||
| variables := make(map[string]Expression) | ||
| variables["w"] = &Integer{5} | ||
| variables["x"] = &Integer{10} | ||
| variables["z"] = &Integer{42} | ||
| result := sentence.Interpret(&variables) | ||
| assert.Equal(t, -27, result) | ||
| } |