feat: new default piece size calculation for transmission-create (#5615)

libtransmission/makemeta.cc

@@ -5,6 +5,7 @@
 
 #include <algorithm>
 #include <cerrno> // for ENOENT
+#include <cmath>
 #include <optional>
 #include <set>
 #include <string>
@@ -376,3 +377,18 @@ std::string tr_metainfo_builder::benc(tr_error** error) const
     tr_variantClear(&top);
     return ret;
 }
+
+uint32_t tr_metainfo_builder::default_piece_size(uint64_t total_size) noexcept
+{
+    TR_ASSERT(total_size != 0);
+
+    // Ideally, we want approximately 2^10 = 1024 pieces, give or take a few hundred pieces.
+    // So we subtract 10 from the log2 of total size.
+    // The ideal number of pieces is up for debate.
+    auto exp = std::log2(total_size) - 10;
+
+    // We want a piece size between 16KiB (2^14 bytes) and 16MiB (2^24 bytes) for maximum compatibility
+    exp = std::clamp(exp, 14., 24.);
+
+    return uint32_t{ 1U } << std::lround(exp);
+}

libtransmission/makemeta.h

@@ -180,44 +180,7 @@ class tr_metainfo_builder
 
     ///
 
-    [[nodiscard]] constexpr static uint32_t default_piece_size(uint64_t total_size) noexcept
-    {
-        uint32_t const KiB = 1024;
-        uint32_t const MiB = 1048576;
-        uint32_t const GiB = 1073741824;
-
-        if (total_size >= 2 * GiB)
-        {
-            return 2 * MiB;
-        }
-
-        if (total_size >= 1 * GiB)
-        {
-            return 1 * MiB;
-        }
-
-        if (total_size >= 512 * MiB)
-        {
-            return 512 * KiB;
-        }
-
-        if (total_size >= 350 * MiB)
-        {
-            return 256 * KiB;
-        }
-
-        if (total_size >= 150 * MiB)
-        {
-            return 128 * KiB;
-        }
-
-        if (total_size >= 50 * MiB)
-        {
-            return 64 * KiB;
-        }
-
-        return 32 * KiB; /* less than 50 meg */
-    }
+    [[nodiscard]] static uint32_t default_piece_size(uint64_t total_size) noexcept;
 
     [[nodiscard]] constexpr static bool is_legal_piece_size(uint32_t x)
     {