Kucoin: Add subscription templating and various fixes (#1579)

* Currency: Variadic Pairs.Add

This version of Pairs.Add is simpler and [more
performant](https://gist.github.com/gbjk/06a1fc1832d04ee41213ca518938cf74)

Behavioural difference: If there's nothing to add, the same slice is
returned unaltered. This seems like good sauce

* Currency: Variadic Remove

* Common: Add Batch function

* Common: Add common.SortStrings for stringers

* Subscriptions: Add batching to templates

* Subscriptions: Sort list of pairs

* Kucoin: Switch to sub templating

* Kucoin: Simplify channel prefix usage

* Kucoin: Fix race on fetchedFuturesOrderbook

* Subscriptions: Filter AssetPairs

Now only the assetPairs relevant to the subscription are in the context

* Subscriptions: Respect subscription Pairs

* Subscriptions: Trim AssetSeparator early

We want to trim before checking for "AssetSeparator vs All" because a
template should be allowed to reuse a range template and generate just one trailing AssetSeparator
whilst using a specific Asset

* Kucoin: Fix empty margin asset added

* Kucoin: Add Subscription batching

Turns out that contary to the documentation, kucoin supports batching of
all symbols and currencies

* Kucoin: Fix checkSubscriptions and coverage

* Subscriptions: Simplify error checking

This reduces the complexity of error checking to just be "do we get the
correct numbers".

Fixes Asset.All with only one asset erroring on xpandPairs, because we
trimmed the only asset separator, and then errored that we're not
xpanding Assets and the asset on the sub is asset.All

This use-case conflicted with commit 6bbd546d74, which required:
```
Subscriptions: Trim AssetSeparator early

We want to trim before checking for "AssetSeparator vs All" because a
template should be allowed to reuse a range template and generate just one trailing AssetSeparator
whilst using a specific Asset
```

Now we set up the assets earlier, and we remove the check for xpandAssets, since the number of asset lines matching is all that matters.

I've removed the asset tests for this, but they were correctly erroring
on the number of asset lines instead.

Everything hits coverage, as well.

* Kucoin: Remove deprecated fundingBook endpoint

* BTCMarkets: Use common.Batch

common/common.go

@@ -15,6 +15,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -388,20 +389,6 @@ func ChangePermission(directory string) error {
 	})
 }
 
-// SplitStringSliceByLimit splits a slice of strings into slices by input limit and returns a slice of slice of strings
-func SplitStringSliceByLimit(in []string, limit uint) [][]string {
-	var stringSlice []string
-	sliceSlice := make([][]string, 0, len(in)/int(limit)+1)
-	for len(in) >= int(limit) {
-		stringSlice, in = in[:limit], in[limit:]
-		sliceSlice = append(sliceSlice, stringSlice)
-	}
-	if len(in) > 0 {
-		sliceSlice = append(sliceSlice, in)
-	}
-	return sliceSlice
-}
-
 // AddPaddingOnUpperCase adds padding to a string when detecting an upper case letter. If
 // there are multiple upper case items like `ThisIsHTTPExample`, it will only
 // pad between like this `This Is HTTP Example`.
@@ -653,3 +640,34 @@ func GetTypeAssertError(required string, received interface{}, fieldDescription
 	}
 	return fmt.Errorf("%w from %T to %s%s", ErrTypeAssertFailure, received, required, description)
 }
+
+// Batch takes a slice type and converts it into a slice of containing slices of length batchSize, and any remainder in the final batch
+// batchSize <= 0 will return the entire input slice in one batch
+func Batch[S ~[]E, E any](blobs S, batchSize int) []S {
+	if len(blobs) == 0 {
+		return []S{}
+	}
+	blobs = slices.Clone(blobs)
+	if batchSize <= 0 {
+		return []S{blobs}
+	}
+	i := 0
+	batches := make([]S, (len(blobs)+batchSize-1)/batchSize)
+	for batchSize < len(blobs) {
+		blobs, batches[i] = blobs[batchSize:], blobs[:batchSize:batchSize]
+		i++
+	}
+	if len(blobs) > 0 {
+		batches[i] = blobs
+	}
+	return batches
+}
+
+// SortStrings takes a slice of fmt.Stringer implementers and returns a new ascending sorted slice
+func SortStrings[S ~[]E, E fmt.Stringer](x S) S {
+	n := slices.Clone(x)
+	slices.SortFunc(n, func(a, b E) int {
+		return strings.Compare(a.String(), b.String())
+	})
+	return n
+}

common/common_test.go

@@ -565,26 +565,6 @@ func initStringSlice(size int) (out []string) {
 	return
 }
 
-func TestSplitStringSliceByLimit(t *testing.T) {
-	t.Parallel()
-	slice50 := initStringSlice(50)
-	out := SplitStringSliceByLimit(slice50, 20)
-	if len(out) != 3 {
-		t.Errorf("expected len() to be 3 instead received: %v", len(out))
-	}
-	if len(out[0]) != 20 {
-		t.Errorf("expected len() to be 20 instead received: %v", len(out[0]))
-	}
-
-	out = SplitStringSliceByLimit(slice50, 50)
-	if len(out) != 1 {
-		t.Errorf("expected len() to be 3 instead received: %v", len(out))
-	}
-	if len(out[0]) != 50 {
-		t.Errorf("expected len() to be 20 instead received: %v", len(out[0]))
-	}
-}
-
 func TestAddPaddingOnUpperCase(t *testing.T) {
 	t.Parallel()
 
@@ -856,3 +836,36 @@ func TestErrorCollector(t *testing.T) {
 	require.True(t, ok, "Must return a multiError")
 	assert.Len(t, errs.Unwrap(), 2, "Should have 2 errors")
 }
+
+// TestBatch ensures the Batch function does not regress into common behavioural faults if implementation changes
+func TestBatch(t *testing.T) {
+	s := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+	b := Batch(s, 3)
+	require.Len(t, b, 4)
+	assert.Len(t, b[0], 3)
+	assert.Len(t, b[3], 1)
+
+	b[0][0] = 42
+	assert.Equal(t, 1, s[0], "Changing the batches must not change the source")
+
+	require.NotPanics(t, func() { Batch(s, -1) }, "Must not panic on negative batch size")
+	done := make(chan any, 1)
+	go func() { done <- Batch(s, 0) }()
+	require.Eventually(t, func() bool { return len(done) > 0 }, time.Second, time.Millisecond, "Batch 0 must not hang")
+
+	for _, i := range []int{-1, 0, 50} {
+		b = Batch(s, i)
+		require.Lenf(t, b, 1, "A batch size of %v should produce a single batch", i)
+		assert.Lenf(t, b[0], len(s), "A batch size of %v should produce a single batch", i)
+	}
+}
+
+type A int
+
+func (a A) String() string {
+	return strconv.Itoa(int(a))
+}
+
+func TestSortStrings(t *testing.T) {
+	assert.Equal(t, []A{1, 2, 5, 6}, SortStrings([]A{6, 2, 5, 1}))
+}