feat(libc: printf): add support for precision specifier

The logs issued by uACPI make plenty use of this field, it is
now necessary to support it.

lib/libc/src/printf.c

@@ -50,6 +50,14 @@ typedef struct {
     } sign_character;
 } flags_t;
 
+/// man 3 printf: Precision
+typedef struct {
+    bool present; // This field is optional
+    bool invalid;
+    int precision;
+    int argument;
+} precision_t;
+
 /// man 3 printf: Length modifier
 typedef struct {
     bool invalid;
@@ -64,7 +72,7 @@ typedef struct {
     flags_t flags;
     length_modifier_t length;
     unsigned int field_with; ///< man 3 printf: Field width
-    // Missing: precision
+    precision_t precision;
 } printf_ctx_t;
 
 static inline bool __attribute__((always_inline)) isdigit(char c)
@@ -225,6 +233,59 @@ static unsigned int printf_field_width(const char *format, int *index)
     return width;
 }
 
+static precision_t printf_precision(const char *format, int *index)
+{
+    precision_t precision = {
+        .present = false,
+        .invalid = false,
+        .precision = 0,
+    };
+
+    if (format[*index] == '.')
+        return precision;
+
+    precision.present = true;
+    *index += 1;
+
+    // A negative precision is taken as if the precision were omitted
+    bool ignore = false;
+
+    switch (format[*index]) {
+
+    case '*':
+        // the actual logic is not implemented, but we need to parse it anyway
+        precision.present = false;
+
+        *index += 1;
+        if (isdigit(format[*index])) {
+            precision.argument = printf_field_width(format, index);
+            if (format[*(index++)] != '$')
+                precision.invalid = true;
+        }
+        break;
+
+    case '-':
+        ignore = true;
+        *index += 1;
+        // cannot have '%.-d' for example
+        if (!isdigit(format[*index])) {
+            precision.invalid = true;
+            return precision;
+        }
+
+        __attribute__((fallthrough));
+
+    default:
+        precision.precision = printf_field_width(format, index);
+        break;
+    }
+
+    if (ignore)
+        precision.precision = 0;
+
+    return precision;
+}
+
 static length_modifier_t printf_length_modifiers(const char *format, int *index)
 {
     length_modifier_t length = {0};
@@ -374,20 +435,22 @@ int vprintf(const char *format, va_list parameters)
 
         // After the delimiter, the argument is of the following format:
         //
-        // %[flag_characters][field_with][length_modifier]conversion_specifier
+        // %[flag_characters][field_with][.precision][length_modifier]conversion_specifier
         //
         // Refer to man 3 printf and the corresponding parsers for more
         // detailed explanations.
 
         const flags_t flags = printf_flags_characters(format, &i);
         const unsigned int width = printf_field_width(format, &i);
+        const precision_t precision = printf_precision(format, &i);
         const length_modifier_t length = printf_length_modifiers(format, &i);
 
         const printf_ctx_t ctx = (printf_ctx_t){
-            .invalid = flags.invalid || length.invalid,
+            .invalid = flags.invalid || length.invalid || precision.invalid,
             .flags = flags,
             .length = length,
             .field_with = width,
+            .precision = precision,
         };
 
         if (ctx.invalid) {