good good study, day day up!

vendor/gopl.io/ch7/surface/surface.go

@@ -0,0 +1,112 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 203.
+
+// The surface program plots the 3-D surface of a user-provided function.
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"math"
+	"net/http"
+)
+
+//!+parseAndCheck
+import "gopl.io/ch7/eval"
+
+//!-parseAndCheck
+
+// -- copied from gopl.io/ch3/surface --
+
+const (
+	width, height = 600, 320            // canvas size in pixels
+	cells         = 100                 // number of grid cells
+	xyrange       = 30.0                // x, y axis range (-xyrange..+xyrange)
+	xyscale       = width / 2 / xyrange // pixels per x or y unit
+	zscale        = height * 0.4        // pixels per z unit
+)
+
+var sin30, cos30 = 0.5, math.Sqrt(3.0 / 4.0) // sin(30°), cos(30°)
+
+func corner(f func(x, y float64) float64, i, j int) (float64, float64) {
+	// find point (x,y) at corner of cell (i,j)
+	x := xyrange * (float64(i)/cells - 0.5)
+	y := xyrange * (float64(j)/cells - 0.5)
+
+	z := f(x, y) // compute surface height z
+
+	// project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy)
+	sx := width/2 + (x-y)*cos30*xyscale
+	sy := height/2 + (x+y)*sin30*xyscale - z*zscale
+	return sx, sy
+}
+
+func surface(w io.Writer, f func(x, y float64) float64) {
+	fmt.Fprintf(w, "<svg xmlns='http://www.w3.org/2000/svg' "+
+		"style='stroke: grey; fill: white; stroke-width: 0.7' "+
+		"width='%d' height='%d'>", width, height)
+	for i := 0; i < cells; i++ {
+		for j := 0; j < cells; j++ {
+			ax, ay := corner(f, i+1, j)
+			bx, by := corner(f, i, j)
+			cx, cy := corner(f, i, j+1)
+			dx, dy := corner(f, i+1, j+1)
+			fmt.Fprintf(w, "<polygon points='%g,%g %g,%g %g,%g %g,%g'/>\n",
+				ax, ay, bx, by, cx, cy, dx, dy)
+		}
+	}
+	fmt.Fprintln(w, "</svg>")
+}
+
+// -- main code for gopl.io/ch7/surface --
+
+//!+parseAndCheck
+func parseAndCheck(s string) (eval.Expr, error) {
+	if s == "" {
+		return nil, fmt.Errorf("empty expression")
+	}
+	expr, err := eval.Parse(s)
+	if err != nil {
+		return nil, err
+	}
+	vars := make(map[eval.Var]bool)
+	if err := expr.Check(vars); err != nil {
+		return nil, err
+	}
+	for v := range vars {
+		if v != "x" && v != "y" && v != "r" {
+			return nil, fmt.Errorf("undefined variable: %s", v)
+		}
+	}
+	return expr, nil
+}
+
+//!-parseAndCheck
+
+//!+plot
+func plot(w http.ResponseWriter, r *http.Request) {
+	r.ParseForm()
+	expr, err := parseAndCheck(r.Form.Get("expr"))
+	if err != nil {
+		http.Error(w, "bad expr: "+err.Error(), http.StatusBadRequest)
+		return
+	}
+	w.Header().Set("Content-Type", "image/svg+xml")
+	surface(w, func(x, y float64) float64 {
+		r := math.Hypot(x, y) // distance from (0,0)
+		return expr.Eval(eval.Env{"x": x, "y": y, "r": r})
+	})
+}
+
+//!-plot
+
+//!+main
+func main() {
+	http.HandleFunc("/plot", plot)
+	log.Fatal(http.ListenAndServe("localhost:8000", nil))
+}
+
+//!-main