From 229d19de63e934947cdb450a90e129be2a535b3b Mon Sep 17 00:00:00 2001
From: Vas Crabb <vas@vastheman.com>
Date: Sat, 24 Feb 2024 02:56:52 +1100
Subject: [PATCH] Revert "util/bitstream.cpp: Fixed cases where bits would be
 dropped when reading and writing. (#12057)"

This reverts commit 69c3cd7daba9e8dd130af167c27ecd8b4131074f.

This causes CHD SHA1 digests to change.  Either it's buggy, or CHD SHA1
digests depend on the representation rather than the data itself.
---
 src/lib/util/bitstream.h | 58 +++++++++-------------------------------
 1 file changed, 13 insertions(+), 45 deletions(-)

diff --git a/src/lib/util/bitstream.h b/src/lib/util/bitstream.h
index 2866d28c20d..4f3817afe6f 100644
--- a/src/lib/util/bitstream.h
+++ b/src/lib/util/bitstream.h
@@ -40,11 +40,10 @@ public:
 private:
 	// internal state
 	uint32_t          m_buffer;       // current bit accumulator
-	int               m_bits;         // number of bits in the accumulator
+	int             m_bits;         // number of bits in the accumulator
 	const uint8_t *   m_read;         // read pointer
 	uint32_t          m_doffset;      // byte offset within the data
 	uint32_t          m_dlength;      // length of the data
-	int               m_dbitoffs;     // bit offset within current read pointer
 };
 
 
@@ -65,7 +64,7 @@ public:
 private:
 	// internal state
 	uint32_t          m_buffer;           // current bit accumulator
-	int               m_bits;             // number of bits in the accumulator
+	int             m_bits;             // number of bits in the accumulator
 	uint8_t *         m_write;            // write pointer
 	uint32_t          m_doffset;          // byte offset within the data
 	uint32_t          m_dlength;          // length of the data
@@ -86,8 +85,7 @@ inline bitstream_in::bitstream_in(const void *src, uint32_t srclength)
 		m_bits(0),
 		m_read(reinterpret_cast<const uint8_t *>(src)),
 		m_doffset(0),
-		m_dlength(srclength),
-		m_dbitoffs(0)
+		m_dlength(srclength)
 {
 }
 
@@ -105,31 +103,12 @@ inline uint32_t bitstream_in::peek(int numbits)
 	// fetch data if we need more
 	if (numbits > m_bits)
 	{
-		while (m_bits < 32)
+		while (m_bits <= 24)
 		{
-			uint32_t newbits = 0;
-
 			if (m_doffset < m_dlength)
-			{
-				// adjust current data to discard any previously read partial bits
-				newbits = (m_read[m_doffset] << m_dbitoffs) & 0xff;
-			}
-
-			if (m_bits + 8 > 32)
-			{
-				// take only what can be used to fill out the rest of the buffer
-				m_dbitoffs = 32 - m_bits;
-				newbits >>= 8 - m_dbitoffs;
-				m_buffer |= newbits;
-				m_bits += m_dbitoffs;
-			}
-			else
-			{
-				m_buffer |= newbits << (24 - m_bits);
-				m_bits += 8 - m_dbitoffs;
-				m_dbitoffs = 0;
-				m_doffset++;
-			}
+				m_buffer |= m_read[m_doffset] << (24 - m_bits);
+			m_doffset++;
+			m_bits += 8;
 		}
 	}
 
@@ -217,11 +196,8 @@ inline bitstream_out::bitstream_out(void *dest, uint32_t destlength)
 
 inline void bitstream_out::write(uint32_t newbits, int numbits)
 {
-	newbits <<= 32 - numbits;
-
 	// flush the buffer if we're going to overflow it
-	while (m_bits + numbits >= 32 && numbits > 0)
-	{
+	if (m_bits + numbits > 32)
 		while (m_bits >= 8)
 		{
 			if (m_doffset < m_dlength)
@@ -231,19 +207,11 @@ inline void bitstream_out::write(uint32_t newbits, int numbits)
 			m_bits -= 8;
 		}
 
-		// offload more bits if it'll still overflow the buffer
-		if (m_bits + numbits >= 32)
-		{
-			const int rem = std::min(32 - m_bits, numbits);
-			m_buffer |= newbits >> m_bits;
-			m_bits += rem;
-			newbits <<= rem;
-			numbits -= rem;
-		}
-	}
-
-	if (numbits <= 0)
-		return;
+	// shift the bits to the top
+	if (numbits == 0)
+		newbits = 0;
+	else
+		newbits <<= 32 - numbits;
 
 	// now shift it down to account for the number of bits we already have and OR them in
 	m_buffer |= newbits >> m_bits;