string compression: save a few bits per string

Length was stored as a 16-bit number always.  Most translations have
a max length far less.  For example, US English translation lengths
always fit in just 8 bits.  probably all languages fit in 9 bits.

This also has the side effect of reducing the alignment of
compressed_string_t from 2 bytes to 1.

testing performed: ran in german and english on pyruler, printed messages
looked right.

Firmware size, en_US
Before: 3044 bytes free in flash
After: 3408 bytes free in flash

Firmware size, de_DE (with #2967 merged to restore translations)
Before: 1236 bytes free in flash
After: 1600 bytes free in flash

2 files changed, 21 insertions, 10 deletions

diff --git a/supervisor/shared/translate.c b/supervisor/shared/translate.c
index 187d5ff8a..606f8fa91 100644
--- a/supervisor/shared/translate.c
+++ b/supervisor/shared/translate.c
@@ -37,7 +37,7 @@
 #include "supervisor/serial.h"
 
 void serial_write_compressed(const compressed_string_t* compressed) {
-    char decompressed[compressed->length];
+    char decompressed[decompress_length(compressed)];
     decompress(compressed, decompressed);
     serial_write(decompressed);
 }
@@ -58,12 +58,22 @@ STATIC int put_utf8(char *buf, int u) {
     }
 }
 
+uint16_t decompress_length(const compressed_string_t* compressed) {
+    if (compress_max_length_bits <= 8) {
+        return 1 + (compressed->data >> (8 - compress_max_length_bits));
+    } else {
+        return 1 + ((compressed->data * 256 + compressed->tail[0]) >> (16 - compress_max_length_bits));
+    }
+}
+
 char* decompress(const compressed_string_t* compressed, char* decompressed) {
-    uint8_t this_byte = 0;
-    uint8_t this_bit = 7;
-    uint8_t b = compressed->data[this_byte];
+    uint8_t this_byte = compress_max_length_bits / 8;
+    uint8_t this_bit = 7 - compress_max_length_bits % 8;
+    uint8_t b = (&compressed->data)[this_byte];
+    uint16_t length = decompress_length(compressed);
+
     // Stop one early because the last byte is always NULL.
-    for (uint16_t i = 0; i < compressed->length - 1;) {
+    for (uint16_t i = 0; i < length - 1;) {
         uint32_t bits = 0;
         uint8_t bit_length = 0;
         uint32_t max_code = lengths[0];
@@ -78,7 +88,7 @@ char* decompress(const compressed_string_t* compressed, char* decompressed) {
             if (this_bit == 0) {
                 this_bit = 7;
                 this_byte += 1;
-                b = compressed->data[this_byte]; // This may read past the end but its never used.
+                b = (&compressed->data)[this_byte]; // This may read past the end but its never used.
             } else {
                 this_bit -= 1;
             }
@@ -91,14 +101,14 @@ char* decompress(const compressed_string_t* compressed, char* decompressed) {
         i += put_utf8(decompressed + i, values[searched_length + bits - max_code]);
     }
 
-    decompressed[compressed->length-1] = '\0';
+    decompressed[length-1] = '\0';
     return decompressed;
 }
 
 inline __attribute__((always_inline)) const compressed_string_t* translate(const char* original) {
     #ifndef NO_QSTR
     #define QDEF(id, str)
-    #define TRANSLATION(id, len, compressed...) if (strcmp(original, id) == 0) { static const compressed_string_t v = {.length = len, .data = compressed}; return &v; } else
+    #define TRANSLATION(id, firstbyte, ...) if (strcmp(original, id) == 0) { static const compressed_string_t v = { .data = firstbyte, .tail = { __VA_ARGS__ } }; return &v; } else
     #include "genhdr/qstrdefs.generated.h"
     #undef TRANSLATION
     #undef QDEF
diff --git a/supervisor/shared/translate.h b/supervisor/shared/translate.h
index 5e8acbb6a..067a44e1f 100644
--- a/supervisor/shared/translate.h
+++ b/supervisor/shared/translate.h
@@ -30,12 +30,13 @@
 #include <stdint.h>
 
 typedef struct {
-    uint16_t length;
-    const uint8_t data[];
+    uint8_t data;
+    const uint8_t tail[];
 } compressed_string_t;
 
 const compressed_string_t* translate(const char* c);
 void serial_write_compressed(const compressed_string_t* compressed);
 char* decompress(const compressed_string_t* compressed, char* decompressed);
+uint16_t decompress_length(const compressed_string_t* compressed);
 
 #endif  // MICROPY_INCLUDED_SUPERVISOR_TRANSLATE_H