1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
|
package main
import (
"github.com/badgerodon/peg"
"fmt"
"strconv"
)
type (
op struct {
val int
op int
next *op
}
)
var (
prec = []int{'*','/','%','+','-'}
ops = map[int]func(int,int)int{
'*': func(a,b int) int {
return a * b
},
'/': func(a,b int) int {
return a / b
},
'%': func(a,b int) int {
return a % b
},
'+': func(a,b int) int {
return a + b
},
'-': func(a,b int) int {
return a - b
},
}
)
func main() {
parser := peg.NewParser()
start := parser.NonTerminal("Start")
expr := parser.NonTerminal("Expression")
paren := parser.NonTerminal("Parentheses")
number := parser.NonTerminal("Number")
start.Expression = expr
expr.Expression = parser.Sequence(
parser.OrderedChoice(
paren,
number,
),
parser.Optional(
parser.Sequence(
parser.OrderedChoice(
parser.Terminal('-'),
parser.Terminal('+'),
parser.Terminal('*'),
parser.Terminal('/'),
),
expr,
),
),
)
paren.Expression = parser.Sequence(
parser.Terminal('('),
expr,
parser.Terminal(')'),
)
number.Expression = parser.ZeroOrMore(
parser.Range('0','9'),
)
tree := parser.Parse("1+1+3*11-5+(2+3)*2/20")
fmt.Println(tree)
fmt.Println(reduce(tree))
}
func reduce(tree *peg.ExpressionTree) int {
// If we're at a number just parse it
if tree.Name == "Number" {
str := ""
for _, c := range tree.Children {
str += string(c.Value)
}
i, _ := strconv.Atoi(str)
return i
}
// We have to collapse all sub expressions into a flattened linked list
// of expressions each of which has an operator. We will then execute
// each of the operators in order of precedence.
fst := &op{0,'+',nil}
lst := fst
var visit func(*peg.ExpressionTree)
visit = func(t *peg.ExpressionTree) {
switch t.Name {
case "Expression":
if len(t.Children) > 1 {
nxt := &op{reduce(t.Children[0]),t.Children[1].Value, nil}
lst.next = nxt
lst = nxt
visit(t.Children[2])
return
}
case "Parentheses":
nxt := &op{reduce(t.Children[1]),0,nil}
lst.next = nxt
lst = nxt
return
}
if len(t.Children) > 0 {
nxt := &op{reduce(t.Children[0]),0,nil}
lst.next = nxt
lst = nxt
}
}
visit(tree)
// Foreach operator in order of precedence
for _, o := range prec {
cur := fst
for cur.next != nil {
if cur.op == o {
cur.val = ops[o](cur.val, cur.next.val)
cur.op = cur.next.op
cur.next = cur.next.next
} else {
cur = cur.next
}
}
}
return fst.val
}
func (this *op) String() string {
str := ""
if this.op == 0 {
str = "(" + fmt.Sprint(this.val) + ") "
} else {
str = "(" + fmt.Sprint(this.val) + " " + string(this.op) + ") "
}
if this.next != nil {
str += this.next.String()
}
return str
}
|
