Ruby: Mimic printf behavior

pkgs/ruby/ext/default/main.c

@@ -129,16 +129,48 @@ VALUE rb_KernAux_snprintf1(
     const char *const format = StringValueCStr(format_rb);
 
     if (size < 0) rb_raise(rb_eRangeError, "expected non-negative size");
+    if (strlen(format) > 100) rb_raise(rb_eArgError, "invalid format");
 
+    const char *fmt = format;
+
+    while (*fmt && *fmt != '%') ++fmt;
+    if (*(fmt++) != '%') rb_raise(rb_eArgError, "invalid format");
+
+    // Mimic printf behavior.
+    if (*fmt == '0' || *fmt == '-' || *fmt == '+' || *fmt == ' ' ||
+        *fmt == '#')
     {
-        size_t fmt_size = 0, perc_count = 0;
-        for (const char *fmt = format; *fmt; ++fmt, ++fmt_size) {
-            if (*fmt == '%') ++perc_count;
-        }
-        if (fmt_size > 100 || perc_count == 0 || perc_count > 2) {
-            rb_raise(rb_eArgError, "invalid format");
+        ++fmt;
+    }
+    if (*fmt >= '0' && *fmt <= '9') {
+        while (*fmt >= '0' && *fmt <= '9') ++fmt;
+    } else if (*fmt == '*') {
+        ++fmt;
+    }
+    if (*fmt == '.') {
+        ++fmt;
+        if (*fmt >= '0' && *fmt <= '9') {
+            while (*fmt >= '0' && *fmt <= '9') ++fmt;
+        } else if (*fmt == '*') {
+            ++fmt;
         }
     }
+    if (*fmt == 'l') {
+        ++fmt;
+        if (*fmt == 'l') ++fmt;
+    } else if (*fmt == 'h') {
+        ++fmt;
+        if (*fmt == 'h') ++fmt;
+    } else if (*fmt == 't' || *fmt == 'j' || *fmt == 'z' || *fmt == 'z') {
+        ++fmt;
+    }
+
+    const char c = *fmt;
+
+    if (*fmt == '%') ++fmt;
+    while (*fmt) {
+        if (*(fmt++) == '%') rb_raise(rb_eArgError, "invalid format");
+    }
 
     union {
         const char *str;
@@ -149,10 +181,6 @@ VALUE rb_KernAux_snprintf1(
     } __attribute__((packed)) arg = { .str = "" };
 
     if (argc == 3) {
-        const char *fmt = format;
-        while (*(fmt + 1)) ++fmt;
-        const char c = *fmt;
-
         VALUE arg_rb = argv_rb[2];
 
         if (c == 'd' || c == 'i') {

pkgs/ruby/spec/lib/kernaux/snprintf1_spec.rb

@@ -20,6 +20,12 @@ RSpec.describe KernAux, '.snprintf1' do
     specify { expect(snprintf1[0]).to eq '%' }
     specify { expect(snprintf1[1]).to eq 1 }
 
+    context 'with leading and trailing spaces' do
+      let(:format) { '  %%  ' }
+
+      specify { expect(snprintf1[0]).to eq '  %  ' }
+    end
+
     context 'with "%s" format' do
       let(:format) { '%s' }
 
@@ -82,6 +88,12 @@ RSpec.describe KernAux, '.snprintf1' do
     specify { expect(snprintf1[0]).to eq arg }
     specify { expect(snprintf1[1]).to eq arg.size }
 
+    context 'with leading and trailing spaces' do
+      let(:format) { '  %s  ' }
+
+      specify { expect(snprintf1[0]).to eq "  #{arg}  " }
+    end
+
     context 'with "%%" format' do
       let(:format) { '%%' }