added int16_t i2s_available(); it returns the number of samples than can be (#3988)

written before blocking

Author: aguaviva

cores/esp8266/core_esp8266_i2s.c

@@ -65,6 +65,10 @@ bool ICACHE_FLASH_ATTR i2s_is_empty(){
   return (i2s_slc_queue_len >= SLC_BUF_CNT-1);
 }
 
+int16_t ICACHE_FLASH_ATTR i2s_available(){
+	return (SLC_BUF_CNT - i2s_slc_queue_len) * SLC_BUF_LEN;
+}
+
 uint32_t ICACHE_FLASH_ATTR i2s_slc_queue_next_item(){ //pop the top off the queue
   uint8_t i;
   uint32_t item = i2s_slc_queue[0];

cores/esp8266/i2s.h

@@ -48,6 +48,7 @@ bool i2s_write_sample_nb(uint32_t sample);//same as above but does not block whe
 bool i2s_write_lr(int16_t left, int16_t right);//combines both channels and calls i2s_write_sample with the result
 bool i2s_is_full();//returns true if DMA is full and can not take more bytes (overflow)
 bool i2s_is_empty();//returns true if DMA is empty (underflow)
+int16_t i2s_available();// returns the number of samples than can be written before blocking
 
 #ifdef __cplusplus
 }