xz: Fix interaction between preset and custom filter chains.

There was somewhat illogical behavior when --extreme was
specified and mixed with custom filter chains.

Before this commit, "xz -9 --lzma2 -e" was equivalent
to "xz --lzma2". After it is equivalent to "xz -6e"
(all earlier preset options get forgotten when a custom
filter chain is specified and the default preset is 6
to which -e is applied). I find this less illogical.

This also affects the meaning of "xz -9e --lzma2 -7".
Earlier it was equivalent to "xz -7e" (the -e specified
before a custom filter chain wasn't forgotten). Now it
is "xz -7". Note that "xz -7e" still is the same as "xz -e7".

Hopefully very few cared about this in the first place,
so pretty much no one should even notice this change.

Thanks to Conley Moorhous.

src/xz/coder.c

@@ -40,12 +40,7 @@ static io_buf out_buf;
 static uint32_t filters_count = 0;
 
 /// Number of the preset (0-9)
-static uint32_t preset_number = 6;
+static uint32_t preset_number = LZMA_PRESET_DEFAULT;
-
-/// If a preset is used (no custom filter chain) and preset_extreme is true,
-/// a significantly slower compression is used to achieve slightly better
-/// compression ratio.
-static bool preset_extreme = false;
 
 /// Integrity check type
 static lzma_check check;
@@ -63,11 +58,9 @@ coder_set_check(lzma_check new_check)
 }
 
 
-extern void
+static void
-coder_set_preset(uint32_t new_preset)
+forget_filter_chain(void)
 {
-	preset_number = new_preset;
-
 	// Setting a preset makes us forget a possibly defined custom
 	// filter chain.
 	while (filters_count > 0) {
@@ -80,10 +73,21 @@ coder_set_preset(uint32_t new_preset)
 }
 
 
+extern void
+coder_set_preset(uint32_t new_preset)
+{
+	preset_number &= ~LZMA_PRESET_LEVEL_MASK;
+	preset_number |= new_preset;
+	forget_filter_chain();
+	return;
+}
+
+
 extern void
 coder_set_extreme(void)
 {
-	preset_extreme = true;
+	preset_number |= LZMA_PRESET_EXTREME;
+	forget_filter_chain();
 	return;
 }
 
@@ -98,6 +102,12 @@ coder_add_filter(lzma_vli id, void *options)
 	filters[filters_count].options = options;
 	++filters_count;
 
+	// Setting a custom filter chain makes us forget the preset options.
+	// This makes a difference if one specifies e.g. "xz -9 --lzma2 -e"
+	// where the custom filter chain resets the preset level back to
+	// the default 6, making the example equivalent to "xz -6e".
+	preset_number = LZMA_PRESET_DEFAULT;
+
 	return;
 }
 
@@ -134,9 +144,6 @@ coder_set_compression_settings(void)
 		}
 
 		// Get the preset for LZMA1 or LZMA2.
-		if (preset_extreme)
-			preset_number |= LZMA_PRESET_EXTREME;
-
 		if (lzma_lzma_preset(&opt_lzma, preset_number))
 			message_bug();