add Force() to force computation of histogram if not cached

godal.cpp

@@ -791,11 +791,14 @@ size_t godalVSIRead(VSILFILE *f, void *buf, int len, char **errmsg) {
 }
 
 void godalRasterHistogram(cctx *ctx, GDALRasterBandH bnd, double *min, double *max, int *buckets,
-						   unsigned long long **values, int bIncludeOutOfRange, int bApproxOK) {
+						   unsigned long long **values, int bIncludeOutOfRange, int bApproxOK, int bForce) {
 	godalWrap(ctx);
 	CPLErr ret = CE_None;
 	if (*buckets == 0) {
-		ret=GDALGetDefaultHistogramEx(bnd,min,max,buckets,values,1,nullptr,nullptr);
+		ret=GDALGetDefaultHistogramEx(bnd,min,max,buckets,values,bForce,nullptr,nullptr);
+		if(ret == CE_Warning && ctx->errMessage == nullptr) {
+			CPLError(CE_Failure, CPLE_AppDefined, "no cached histogram available");
+		}
 	} else {
 		*values = (unsigned long long*) VSIMalloc(*buckets*sizeof(GUIntBig));
 		ret=GDALGetRasterHistogramEx(bnd,*min,*max,*buckets,*values,bIncludeOutOfRange,bApproxOK,nullptr,nullptr);

godal.go

@@ -421,7 +421,7 @@ func (band Band) Histogram(opts ...HistogramOption) (Histogram, error) {
 	cgc := createCGOContext(nil, hopt.errorHandler)
 
 	C.godalRasterHistogram(cgc.cPointer(), band.handle(), (*C.double)(&hopt.min), (*C.double)(&hopt.max), (*C.int)(&hopt.buckets),
-		&values, C.int(hopt.includeOutside), C.int(hopt.approx))
+		&values, C.int(hopt.includeOutside), C.int(hopt.approx), C.int(hopt.force))
 	if err := cgc.close(); err != nil {
 		return Histogram{}, err
 	}

godal.h

@@ -100,7 +100,7 @@ extern "C" {
 	void godalGetColorTable(GDALRasterBandH bnd, GDALPaletteInterp *interp, int *nEntries, short **entries);
 	void godalSetColorTable(cctx *ctx, GDALRasterBandH bnd, GDALPaletteInterp interp, int nEntries, short *entries);
 	void godalRasterHistogram(cctx *ctx, GDALRasterBandH bnd, double *min, double *max, int *buckets,
-						   unsigned long long **values, int bIncludeOutOfRange, int bApproxOK);
+						   unsigned long long **values, int bIncludeOutOfRange, int bApproxOK, int bForce);
 
 	VSILFILE *godalVSIOpen(cctx *ctx, const char *name);
 	void godalVSIUnlink(cctx *ctx, const char *name);

godal_test.go

@@ -395,9 +395,15 @@ func TestHistogram(t *testing.T) {
 	bnd := ds.Bands()[0]
 
 	_, err := bnd.Histogram()
-	assert.NoError(t, err)
+	assert.Error(t, err)
+	assert.Equal(t, "no cached histogram available", err.Error())
+
 	ehc := eh()
 	hist, err := bnd.Histogram(ErrLogger(ehc.ErrorHandler))
+	assert.Error(t, err)
+	assert.Equal(t, "no cached histogram available", err.Error())
+
+	hist, err = bnd.Histogram(Force())
 	assert.NoError(t, err)
 
 	ll := hist.Len()

histogram.go

@@ -42,8 +42,9 @@ func (h Histogram) Bucket(i int) Bucket {
 }
 
 type histogramOpts struct {
-	approx         int
-	includeOutside int
+	approx         int //bool
+	force          int //bool
+	includeOutside int //bool
 	min, max       float64
 	buckets        int32
 	errorHandler   ErrorHandler
@@ -55,6 +56,9 @@ type histogramOpts struct {
 //  - Approximate() to allow the algorithm to operate on a subset of the full resolution data
 //  - Intervals(count int, min,max float64) to compute a histogram with count buckets, spanning [min,max].
 //   Each bucket will be (max-min)/count wide. If not provided, the default histogram will be returned.
+//  - Force() to allow gdal to scan the full dataset to extract the histogram if no cached version
+//   is available. This option is not used if also using Intervals(). The default (i.e. without using
+//   Force()) is to return an error if a cached histogram is not available.
 //  - IncludeOutOfRange() to populate the first and last bucket with values under/over the specified min/max
 //   when used in conjuntion with Intervals()
 //  - ErrLogger
@@ -89,6 +93,20 @@ func Approximate() interface {
 	return approximateOkOption{}
 }
 
+type forceOption struct{}
+
+func (aoo forceOption) setHistogramOpt(ho *histogramOpts) {
+	ho.force = 1
+}
+
+// Force allows the histogram algorithm to scan the dataset to collect statistics if a cached
+// histogram is not available (false by default, may cause a long computation if set)
+func Force() interface {
+	HistogramOption
+} {
+	return forceOption{}
+}
+
 type intervalsOption struct {
 	min, max float64
 	buckets  int32
@@ -101,7 +119,9 @@ func (io intervalsOption) setHistogramOpt(ho *histogramOpts) {
 }
 
 // Intervals computes a histogram with count buckets, spanning [min,max].
-// Each bucket will be (max-min)/count wide. If not provided, the default histogram will be returned.
+// Each bucket will be (max-min)/count wide.
+// If provided, this option will force a dataset rescan which may be time consuming.
+// If not provided, the default histogram will be returned
 func Intervals(count int, min, max float64) interface {
 	HistogramOption
 } {