Make canvasSize work on windows.

image.go

@@ -3,8 +3,6 @@ package main
 import (
 	"image"
 	"os"
-	"syscall"
-	"unsafe"
 )
 
 const defaultRatio float64 = 7.0 / 3.0 // The terminal's default cursor width/height ratio
@@ -20,22 +18,6 @@ func load(filename string) (image.Image, error) {
 	return img, err
 }
 
-// canvasSize returns the terminal columns, rows, and cursor aspect ratio
-func canvasSize() (int, int, float64) {
-	var size [4]uint16
-	if _, _, err := syscall.Syscall6(syscall.SYS_IOCTL, uintptr(os.Stdout.Fd()), uintptr(syscall.TIOCGWINSZ), uintptr(unsafe.Pointer(&size)), 0, 0, 0); err != 0 {
-		panic(err)
-	}
-	rows, cols, width, height := size[0], size[1], size[2], size[3]
-
-	var whratio = defaultRatio
-	if width > 0 && height > 0 {
-		whratio = float64(height/rows) / float64(width/cols)
-	}
-
-	return int(cols), int(rows), whratio
-}
-
 // scales calculates the image scale to fit within the terminal width/height
 func scale(imgW, imgH, termW, termH int, whratio float64) float64 {
 	hr := float64(imgH) / (float64(termH) * whratio)

size_unix.go

@@ -0,0 +1,36 @@
+// Builds for these work but not all the resultant binaries have been tested.
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+// Note that as of at least termbox-go@v1.1.1 the following GOOS
+// do not build (for termbox):
+//	aix
+//	illumos
+//	plan9
+//	solaris
+// For other reasons, pxl doesn't appear to build on GOOS:
+//	android
+//	ios
+
+package main
+
+import (
+	"os"
+	"syscall"
+	"unsafe"
+)
+
+// canvasSize returns the terminal columns, rows, and cursor aspect ratio
+func canvasSize() (int, int, float64) {
+	var size [4]uint16
+	if _, _, err := syscall.Syscall6(syscall.SYS_IOCTL, uintptr(os.Stdout.Fd()), uintptr(syscall.TIOCGWINSZ), uintptr(unsafe.Pointer(&size)), 0, 0, 0); err != 0 {
+		panic(err)
+	}
+	rows, cols, width, height := size[0], size[1], size[2], size[3]
+
+	var whratio = defaultRatio
+	if width > 0 && height > 0 {
+		whratio = float64(height/rows) / float64(width/cols)
+	}
+
+	return int(cols), int(rows), whratio
+}

size_windows.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"sync"
+	"syscall"
+	"unsafe"
+
+	"github.com/nsf/termbox-go"
+)
+
+// canvasSize returns the terminal columns, rows, and cursor aspect ratio
+func canvasSize() (int, int, float64) {
+	cols, rows := termbox.Size()
+	var whratio = defaultRatio
+	if width, height, err := primarySize(); err == nil && width > 0 && height > 0 {
+		whratio = float64(height/rows) / float64(width/cols)
+	}
+	return cols, rows, whratio
+}
+
+// Extracted (and trimmed down) from
+// https://gist.github.com/dchapes/b90e4e0bde7d2672647b0d479ed42dbe
+// (That's also why there are several functions and a bit more
+//  boiler plate that required for this one user32.dll call).
+
+const spiGetWorkArea = 0x0030
+
+var winapi struct {
+	once                 sync.Once
+	systemParametersInfo uintptr
+	err                  error
+}
+
+func loadWinAPI() {
+	winapi.once.Do(func() {
+		libuser32, err := syscall.LoadLibrary("user32.dll")
+		if err != nil {
+			winapi.err = err
+			return
+		}
+		winapi.systemParametersInfo, winapi.err = syscall.GetProcAddress(libuser32, "SystemParametersInfoW")
+	})
+}
+
+func systemParametersInfo(uiAction, uiParam uint32, pvParam unsafe.Pointer, fWinIni uint32) error {
+	loadWinAPI()
+	if winapi.err != nil {
+		return winapi.err
+	}
+
+	r1, _, err := syscall.Syscall6(winapi.systemParametersInfo, 4,
+		uintptr(uiAction), uintptr(uiParam),
+		uintptr(pvParam), uintptr(fWinIni), 0, 0)
+	if r1 == 0 {
+		return err
+	}
+	return nil
+}
+
+// primarySize returns the size of the primary display monitor.
+func primarySize() (width, height int, err error) {
+	var rect struct {
+		left   uint32
+		top    uint32
+		right  uint32
+		bottom uint32
+		// extra size as a safety margin
+		_ uint32
+		_ uint32
+		_ uint32
+		_ uint32
+	}
+	err = systemParametersInfo(spiGetWorkArea, 0, unsafe.Pointer(&rect.left), 0)
+	if err != nil {
+		return 0, 0, err
+	}
+	return int(rect.right) - int(rect.left),
+		int(rect.bottom) - int(rect.top), nil
+}