feat: add p/demo/entropy (#2487)

- Revert partially "feat(stdlibs): add math/rand (#2455)
(f547d7d)
- Update p/demo/rand and rename to p/demo/entropy

---------

Signed-off-by: moul <94029+moul@users.noreply.github.com>
Co-authored-by: Marc Vertes <mvertes@free.fr>

examples/gno.land/p/demo/entropy/entropy.gno

@@ -0,0 +1,89 @@
+// Entropy generates fully deterministic, cost-effective, and hard to guess
+// numbers.
+//
+// It is designed both for single-usage, like seeding math/rand or for being
+// reused which increases the entropy and its cost effectiveness.
+//
+// Disclaimer: this package is unsafe and won't prevent others to guess values
+// in advance.
+//
+// It uses the Bernstein's hash djb2 to be CPU-cycle efficient.
+package entropy
+
+import (
+	"math"
+	"std"
+	"time"
+)
+
+type Instance struct {
+	value uint32
+}
+
+func New() *Instance {
+	r := Instance{value: 5381}
+	r.addEntropy()
+	return &r
+}
+
+func FromSeed(seed uint32) *Instance {
+	r := Instance{value: seed}
+	r.addEntropy()
+	return &r
+}
+
+func (i *Instance) Seed() uint32 {
+	return i.value
+}
+
+func (i *Instance) djb2String(input string) {
+	for _, c := range input {
+		i.djb2Uint32(uint32(c))
+	}
+}
+
+// super fast random algorithm.
+// http://www.cse.yorku.ca/~oz/hash.html
+func (i *Instance) djb2Uint32(input uint32) {
+	i.value = (i.value << 5) + i.value + input
+}
+
+// AddEntropy uses various runtime variables to add entropy to the existing seed.
+func (i *Instance) addEntropy() {
+	// FIXME: reapply the 5381 initial value?
+
+	// inherit previous entropy
+	// nothing to do
+
+	// handle callers
+	{
+		caller1 := std.GetCallerAt(1).String()
+		i.djb2String(caller1)
+		caller2 := std.GetCallerAt(2).String()
+		i.djb2String(caller2)
+	}
+
+	// height
+	{
+		height := std.GetHeight()
+		if height >= math.MaxUint32 {
+			height -= math.MaxUint32
+		}
+		i.djb2Uint32(uint32(height))
+	}
+
+	// time
+	{
+		secs := time.Now().Second()
+		i.djb2Uint32(uint32(secs))
+		nsecs := time.Now().Nanosecond()
+		i.djb2Uint32(uint32(nsecs))
+	}
+
+	// FIXME: compute other hard-to-guess but deterministic variables, like real gas?
+}
+
+func (i *Instance) Value() uint32 {
+	i.addEntropy()
+	return i.value
+}

examples/gno.land/p/demo/entropy/entropy_test.gno

@@ -0,0 +1,46 @@
+package entropy
+
+import (
+	"std"
+	"strconv"
+	"testing"
+)
+
+func TestInstance(t *testing.T) {
+	instance := New()
+	if instance == nil {
+		t.Errorf("instance should not be nil")
+	}
+}
+
+func TestInstanceValue(t *testing.T) {
+	baseEntropy := New()
+	baseResult := computeValue(t, baseEntropy)
+
+	sameHeightEntropy := New()
+	sameHeightResult := computeValue(t, sameHeightEntropy)
+
+	if baseResult != sameHeightResult {
+		t.Errorf("should have the same result: new=%s, base=%s", sameHeightResult, baseResult)
+	}
+
+	std.TestSkipHeights(1)
+	differentHeightEntropy := New()
+	differentHeightResult := computeValue(t, differentHeightEntropy)
+
+	if baseResult == differentHeightResult {
+		t.Errorf("should have different result: new=%s, base=%s", differentHeightResult, baseResult)
+	}
+}
+
+func computeValue(t *testing.T, r *Instance) string {
+	t.Helper()
+
+	out := ""
+	for i := 0; i < 10; i++ {
+		val := int(r.Value())
+		out += strconv.Itoa(val) + " "
+	}
+
+	return out
+}

examples/gno.land/p/demo/entropy/gno.mod

@@ -0,0 +1 @@
+module gno.land/p/demo/entropy

examples/gno.land/p/demo/entropy/z_filetest.gno

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"std"
+
+	"gno.land/p/demo/entropy"
+)
+
+func main() {
+	// initial
+	println("---")
+	r := entropy.New()
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+
+	// should be the same
+	println("---")
+	r = entropy.New()
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+
+	std.TestSkipHeights(1)
+	println("---")
+	r = entropy.New()
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+	println(r.Value())
+}
+
+// Output:
+// ---
+// 4129293727
+// 2141104956
+// 1950222777
+// 3348280598
+// 438354259
+// ---
+// 4129293727
+// 2141104956
+// 1950222777
+// 3348280598
+// 438354259
+// ---
+// 49506731
+// 1539580078
+// 2695928529
+// 1895482388
+// 3462727799

examples/gno.land/r/demo/art/gnoface/gno.mod

@@ -1,6 +1,7 @@
 module gno.land/r/demo/art/gnoface
 
 require (
+	gno.land/p/demo/entropy v0.0.0-latest
 	gno.land/p/demo/uassert v0.0.0-latest
 	gno.land/p/demo/ufmt v0.0.0-latest
 )

examples/gno.land/r/demo/art/gnoface/gnoface.gno

@@ -2,15 +2,15 @@ package gnoface
 
 import (
 	"math/rand"
-	"std"
 	"strconv"
 	"strings"
 
+	"gno.land/p/demo/entropy"
 	"gno.land/p/demo/ufmt"
 )
 
 func Render(path string) string {
-	seed := uint64(std.GetHeight())
+	seed := uint64(entropy.New().Value())
 
 	path = strings.TrimSpace(path)
 	if path != "" {