Allow controlling num of CPUs for images operations

tools/utils/images/utils.go

@@ -0,0 +1,56 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package images
+
+import (
+	"fmt"
+	"runtime"
+	"sync"
+	"sync/atomic"
+)
+
+var _ = fmt.Print
+
+type Context struct {
+	num_of_threads atomic.Int32
+}
+
+func (self *Context) SetNumberOfThreads(n int) {
+	self.num_of_threads.Store(int32(n))
+}
+
+func (self *Context) NumberOfThreads() int {
+	return int(self.num_of_threads.Load())
+}
+
+// parallel processes the data in separate goroutines.
+func (self *Context) Parallel(start, stop int, fn func(<-chan int)) {
+	count := stop - start
+	if count < 1 {
+		return
+	}
+
+	procs := self.NumberOfThreads()
+	if procs <= 0 {
+		procs = runtime.NumCPU()
+	}
+	if procs > count {
+		procs = count
+	}
+
+	c := make(chan int, count)
+	for i := start; i < stop; i++ {
+		c <- i
+	}
+	close(c)
+
+	var wg sync.WaitGroup
+	for i := 0; i < procs; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			fn(c)
+		}()
+	}
+	wg.Wait()
+}