[libc] Implement strftime

Implements the posix-specified strftime conversions for the default
locale, along with comprehensive unit tests. This reuses a lot of design
from printf, as well as the printf writer.

Roughly based on #111305, but with major rewrites.

Author: michaelrj-google

libc/config/linux/x86_64/entrypoints.txt

@@ -1115,6 +1115,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.gmtime_r
     libc.src.time.mktime
     libc.src.time.nanosleep
+    libc.src.time.strftime
     libc.src.time.time
     libc.src.time.timespec_get
 

libc/docs/dev/undefined_behavior.rst

@@ -106,3 +106,40 @@ uninitialized spinlock and invalid spinlock is left undefined. We follow the rec
 POSIX.1-2024, where EINVAL is returned if the spinlock is invalid (here we only check for null pointers) or
 EBUSY is returned if the spinlock is currently locked. The lock is poisoned after a successful destroy. That is,
 subsequent operations on the lock object without any reinitialization will return EINVAL.
+
+Strftime
+--------
+In the C Standard, it provides a list of modifiers, and the conversions these
+are valid on. It also says that a modifier on an unspecified conversion is
+undefined. For LLVM-libc, the conversion is treated as if the modifier isn't
+there.
+
+If a struct tm with values out of the normal range is passed, the standard says
+the result is undefined. For LLVM-libc, the result may be either the normalized
+value (e.g. weekday % 7) or the actual, out of range value. For any numeric
+conversion where the result is just printing a value out of the struct
+(e.g. "%w" prints the day of the week), no normalization occurs ("%w" on a
+tm_wday of 32 prints "32"). For any numeric conversion where the value is
+calculated (e.g. "%u" prints the day of the week, starting on monday), the
+value is normalized (e.g. "%u" on a tm_wday of 32 prints "4"). For conversions
+that result in strings, passing an out of range value will result in "?".
+
+Posix adds padding support to strftime, but says "the default padding character
+is unspecified." For LLVM-libc, the default padding character is ' ' (space)
+for all string-type conversions and '0' for integer-type conversions. Composite
+conversions pass the padding to the first (leftmost) conversion. In practice
+this is always a numeric conversion, so it pads with '0'. For the purposes of
+padding, composite conversions also assume the non-leading conversions have
+valid inputs and output their expected number of characters. For %c this means
+that the padding will be off if the year is outside of the range -999 to 9999.
+
+The %e conversion is padded with spaces by default, but pads with 0s if the '0'
+flag is set.
+
+Posix also adds flags and a minimum field width, but leaves unspecified what
+happens for most combinations of these. For LLVM-libc:
+An unspecified minimum field width defaults to 0.
+More specific flags take precedence over less specific flags (i.e. '+' takes precedence over '0')
+Any conversion with a minimum width is padded with the padding character until it is at least as long as the minimum width.
+Modifiers are applied, then the result is padded if necessary.
+Any composite conversion will pass along all flags to the component conversions.

libc/include/time.yaml

@@ -91,6 +91,25 @@ functions:
     arguments:
       - type: const struct timespec *
       - type: struct timespec *
+  - name: strftime
+    standard:
+      - stdc
+    return_type: size_t
+    arguments:
+      - type: char *__restrict
+      - type: size_t
+      - type: const char *__restrict
+      - type: const struct tm *__restrict
+  - name: strftime_l
+    standard:
+      - stdc
+    return_type: size_t
+    arguments:
+      - type: char *__restrict
+      - type: size_t
+      - type: const char *__restrict
+      - type: const struct tm *__restrict
+      - type: locale_t
   - name: time
     standard:
       - stdc

libc/src/time/CMakeLists.txt

@@ -135,6 +135,21 @@ add_entrypoint_object(
     libc.hdr.types.struct_tm
 )
 
+add_subdirectory(strftime_core) #TODO: Move to top
+
+add_entrypoint_object(
+  strftime
+  SRCS
+    strftime.cpp
+  HDRS
+    strftime.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.hdr.types.struct_tm
+    libc.src.stdio.printf_core.writer
+    libc.src.time.strftime_core.strftime_main
+)
+
 add_entrypoint_object(
   time
   SRCS

libc/src/time/strftime.cpp

@@ -0,0 +1,31 @@
+//===-- Implementation of strftime function -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/strftime.h"
+#include "hdr/types/size_t.h"
+#include "hdr/types/struct_tm.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/stdio/printf_core/writer.h"
+#include "src/time/strftime_core/strftime_main.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(size_t, strftime,
+                   (char *__restrict buffer, size_t buffsz,
+                    const char *__restrict format, const struct tm *timeptr)) {
+
+  printf_core::WriteBuffer wb(buffer, (buffsz > 0 ? buffsz - 1 : 0));
+  printf_core::Writer writer(&wb);
+  int ret = strftime_core::strftime_main(&writer, format, timeptr);
+  if (buffsz > 0) // if the buffsz is 0 the buffer may be a null pointer.
+    wb.buff[wb.buff_cur] = '\0';
+  return (ret < 0 || static_cast<size_t>(ret) > buffsz) ? 0 : ret;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/time/strftime.h

@@ -0,0 +1,23 @@
+//===-- Implementation header of strftime -----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_STRFTIME_H
+#define LLVM_LIBC_SRC_TIME_STRFTIME_H
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/struct_tm.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+size_t strftime(char *__restrict, size_t max, const char *__restrict format,
+                const struct tm *timeptr);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_TIME_STRFTIME_H

libc/src/time/strftime_core/CMakeLists.txt

@@ -0,0 +1,51 @@
+add_header_library(
+  core_structs
+  HDRS
+    core_structs.h
+  DEPENDS
+    libc.src.__support.CPP.string_view
+    libc.hdr.types.struct_tm
+)
+
+add_header_library(
+  parser
+  HDRS
+    parser.h
+  DEPENDS
+    .core_structs
+    libc.src.__support.CPP.string_view
+    libc.src.__support.ctype_utils
+    libc.src.__support.str_to_integer
+)
+
+add_object_library(
+  converter
+  SRCS
+    converter.cpp
+  HDRS
+    converter.h
+    num_converter.h
+    str_converter.h
+    composite_converter.h
+  DEPENDS
+    .core_structs
+    libc.src.time.time_utils
+    libc.src.time.time_constants
+    libc.src.stdio.printf_core.writer
+    libc.src.__support.CPP.string_view
+    libc.src.__support.integer_to_string
+)
+
+add_object_library(
+  strftime_main
+  SRCS
+    strftime_main.cpp
+  HDRS
+    strftime_main.h
+  DEPENDS
+    .core_structs
+    .parser
+    .converter
+    libc.src.stdio.printf_core.writer
+    libc.hdr.types.struct_tm
+)