mirror of
https://github.com/gopl-zh/gopl-zh.github.com.git
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113 lines
2.7 KiB
Go
113 lines
2.7 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 203.
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// The surface program plots the 3-D surface of a user-provided function.
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package main
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import (
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"fmt"
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"io"
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"log"
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"math"
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"net/http"
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)
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//!+parseAndCheck
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import "gopl.io/ch7/eval"
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//!-parseAndCheck
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// -- copied from gopl.io/ch3/surface --
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const (
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width, height = 600, 320 // canvas size in pixels
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cells = 100 // number of grid cells
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xyrange = 30.0 // x, y axis range (-xyrange..+xyrange)
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xyscale = width / 2 / xyrange // pixels per x or y unit
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zscale = height * 0.4 // pixels per z unit
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)
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var sin30, cos30 = 0.5, math.Sqrt(3.0 / 4.0) // sin(30°), cos(30°)
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func corner(f func(x, y float64) float64, i, j int) (float64, float64) {
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// find point (x,y) at corner of cell (i,j)
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x := xyrange * (float64(i)/cells - 0.5)
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y := xyrange * (float64(j)/cells - 0.5)
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z := f(x, y) // compute surface height z
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// project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy)
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sx := width/2 + (x-y)*cos30*xyscale
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sy := height/2 + (x+y)*sin30*xyscale - z*zscale
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return sx, sy
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}
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func surface(w io.Writer, f func(x, y float64) float64) {
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fmt.Fprintf(w, "<svg xmlns='http://www.w3.org/2000/svg' "+
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"style='stroke: grey; fill: white; stroke-width: 0.7' "+
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"width='%d' height='%d'>", width, height)
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for i := 0; i < cells; i++ {
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for j := 0; j < cells; j++ {
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ax, ay := corner(f, i+1, j)
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bx, by := corner(f, i, j)
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cx, cy := corner(f, i, j+1)
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dx, dy := corner(f, i+1, j+1)
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fmt.Fprintf(w, "<polygon points='%g,%g %g,%g %g,%g %g,%g'/>\n",
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ax, ay, bx, by, cx, cy, dx, dy)
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}
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}
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fmt.Fprintln(w, "</svg>")
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}
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// -- main code for gopl.io/ch7/surface --
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//!+parseAndCheck
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func parseAndCheck(s string) (eval.Expr, error) {
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if s == "" {
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return nil, fmt.Errorf("empty expression")
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}
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expr, err := eval.Parse(s)
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if err != nil {
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return nil, err
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}
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vars := make(map[eval.Var]bool)
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if err := expr.Check(vars); err != nil {
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return nil, err
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}
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for v := range vars {
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if v != "x" && v != "y" && v != "r" {
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return nil, fmt.Errorf("undefined variable: %s", v)
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}
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}
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return expr, nil
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}
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//!-parseAndCheck
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//!+plot
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func plot(w http.ResponseWriter, r *http.Request) {
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r.ParseForm()
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expr, err := parseAndCheck(r.Form.Get("expr"))
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if err != nil {
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http.Error(w, "bad expr: "+err.Error(), http.StatusBadRequest)
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return
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}
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w.Header().Set("Content-Type", "image/svg+xml")
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surface(w, func(x, y float64) float64 {
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r := math.Hypot(x, y) // distance from (0,0)
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return expr.Eval(eval.Env{"x": x, "y": y, "r": r})
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})
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}
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//!-plot
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//!+main
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func main() {
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http.HandleFunc("/plot", plot)
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log.Fatal(http.ListenAndServe("localhost:8000", nil))
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}
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//!-main
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