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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

// See page 198.

// Package eval provides an expression evaluator.
package eval

import (
    "fmt"
    "math"
)

//!+env

type Env map[Var]float64

//!-env

//!+Eval1

func (v Var) Eval(env Env) float64 {
    return env[v]
}

func (l literal) Eval(_ Env) float64 {
    return float64(l)
}

//!-Eval1

//!+Eval2

func (u unary) Eval(env Env) float64 {
    switch u.op {
    case '+':
        return +u.x.Eval(env)
    case '-':
        return -u.x.Eval(env)
    }
    panic(fmt.Sprintf("unsupported unary operator: %q", u.op))
}

func (b binary) Eval(env Env) float64 {
    switch b.op {
    case '+':
        return b.x.Eval(env) + b.y.Eval(env)
    case '-':
        return b.x.Eval(env) - b.y.Eval(env)
    case '*':
        return b.x.Eval(env) * b.y.Eval(env)
    case '/':
        return b.x.Eval(env) / b.y.Eval(env)
    }
    panic(fmt.Sprintf("unsupported binary operator: %q", b.op))
}

func (c call) Eval(env Env) float64 {
    switch c.fn {
    case "pow":
        return math.Pow(c.args[0].Eval(env), c.args[1].Eval(env))
    case "sin":
        return math.Sin(c.args[0].Eval(env))
    case "sqrt":
        return math.Sqrt(c.args[0].Eval(env))
    }
    panic(fmt.Sprintf("unsupported function call: %s", c.fn))
}

//!-Eval2