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71 lines
1.2 KiB
Go
71 lines
1.2 KiB
Go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 198.
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// Package eval provides an expression evaluator.
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package eval
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import (
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"fmt"
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"math"
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)
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//!+env
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type Env map[Var]float64
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//!-env
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//!+Eval1
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func (v Var) Eval(env Env) float64 {
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return env[v]
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}
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func (l literal) Eval(_ Env) float64 {
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return float64(l)
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}
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//!-Eval1
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//!+Eval2
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func (u unary) Eval(env Env) float64 {
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switch u.op {
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case '+':
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return +u.x.Eval(env)
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case '-':
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return -u.x.Eval(env)
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}
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panic(fmt.Sprintf("unsupported unary operator: %q", u.op))
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}
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func (b binary) Eval(env Env) float64 {
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switch b.op {
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case '+':
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return b.x.Eval(env) + b.y.Eval(env)
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case '-':
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return b.x.Eval(env) - b.y.Eval(env)
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case '*':
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return b.x.Eval(env) * b.y.Eval(env)
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case '/':
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return b.x.Eval(env) / b.y.Eval(env)
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}
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panic(fmt.Sprintf("unsupported binary operator: %q", b.op))
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}
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func (c call) Eval(env Env) float64 {
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switch c.fn {
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case "pow":
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return math.Pow(c.args[0].Eval(env), c.args[1].Eval(env))
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case "sin":
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return math.Sin(c.args[0].Eval(env))
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case "sqrt":
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return math.Sqrt(c.args[0].Eval(env))
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}
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panic(fmt.Sprintf("unsupported function call: %s", c.fn))
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}
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//!-Eval2
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