feat: adds tip004 pow changes

src/RFC/MinoTari/TIP-RFC-MT-0004_MinoTari_PoW_Difficulty_changes.md

@@ -0,0 +1,157 @@
+# A-TIP-RFC-MT-0004: MinoTari PoW difficulty changes
+
+| TIP             | [A-TIP-RFC-MT-0004](#/RFC/MinoTari/TIP-RFC-MT-0004_MinoTari_PoW_Difficulty_changes.md)                  |
+|-----------------|---------------------------------------------------------------------------|
+| Title           | MinoTari PoW difficulty changes                                           |
+| Last Modified   | 2026-05-17                                                                |
+| Authors         | SW van Heerden                                                            |
+| Status          | Accepted                                                                  |
+| Type            | Process                                                                   |
+| Created         | 2026-06-11                                                                |
+| References      |                                                                           |
+
+## Overview
+
+Tari Core currently uses a geometric-mean-style comparison to determine
+the best chain tip across multiple Proof-of-Work (PoW) algorithms.
+
+The geometric mean is a measure of central tendency calculated by
+multiplying a set of values together and then taking the n-th root
+(where n is the number of values). Unlike the arithmetic mean, the
+geometric mean is well suited to comparing values that span multiple
+orders of magnitude.
+
+Tari does not compute the full geometric mean. Specifically, the final
+n-th root step is omitted to avoid floating-point operations. Since the
+protocol only needs to compare two values (rather than compute the exact
+geometric mean), the root operation is unnecessary --- multiplying the
+accumulated difficulties is sufficient for ordering purposes.
+
+Tari currently supports four PoW algorithms:
+
+-   Rx
+-   RxT
+-   Sha3x
+-   C29
+
+To compare two competing chain tips, Tari calculates:
+
+    Rx * RxT * Sha3x * C29
+
+The chain with the larger product is considered to have more accumulated
+PoW.
+
+### Example
+
+Assume the total accumulated difficulties for the four algorithms are:
+
+    Rx, RxT, Sha3x, C29
+
+Now consider two competing new blocks:
+
+-   One mined on Rx with difficulty `x`
+-   One mined on Sha3x with difficulty `y`
+
+We compare:
+
+    (Rx + x) * RxT * Sha3x * C29
+    Rx * RxT * (Sha3x + y) * C29
+
+After cancelling common terms, this comparison reduces to evaluating:
+
+    x / Rx  vs  y / Sha3x
+
+Whichever ratio is larger represents the larger relative increase in
+accumulated PoW.
+
+### Problem
+
+This approach works under balanced hash rate conditions. However, if one
+algorithm (e.g., Sha3x) experiences a significant hash rate increase ---
+for example due to ASIC hardware becoming dominant relative to GPU
+mining --- its relative contribution can dominate the comparison.
+
+As accumulated difficulty grows, terms such as `x / Rx` converge toward
+zero unless `x` scales proportionally with `Rx`. If one algorithm's
+difficulty grows much faster than the others, its blocks will
+consistently produce larger relative increases.
+
+This creates a situation where one algorithm can disproportionately
+reorg blocks mined by other algorithms. That outcome undermines the
+original design goal of including multiple PoW algorithms for
+decentralization and hardware diversity.
+
+
+## Proposed Change
+
+Tari currently calculates target difficulty using LWMA (Linearly
+Weighted Moving Average) over the last 90 blocks. The LWMA uses:
+
+-   Target block time
+-   Header timestamps
+-   Historical target difficulties
+
+to compute the next target difficulty.
+
+
+Introduce an exponential backoff mechanism for consecutive blocks mined
+by the same PoW algorithm:
+
+-   If a block is mined using algorithm A, and the previous block was
+    also mined using algorithm A, then the target time for algorithm A
+    is doubled.
+-   This doubling continues for each consecutive block of the same
+    algorithm.
+-   If a block of a different algorithm is mined, the target time for
+    algorithm A resets to its base consensus value.
+
+### Example
+
+Assume the base target time for Sha3x is **8 minutes**.
+
+-   If the previous block was Sha3x → the next Sha3x target time becomes
+    **16 minutes**.
+-   If another Sha3x block is mined consecutively → target time becomes
+    **32 minutes**.
+-   If a different algorithm mines a block → Sha3x target time resets to
+    **8 minutes**.
+
+This mechanism:
+
+-   Does not alter the underlying hash rate.
+-   Does not directly manipulate accumulated difficulty.
+-   Makes selfish mining exponentially more expensive for any single
+    algorithm attempting to dominate.
+-   Encourages natural interleaving of algorithms.
+-   
+## Consequences
+
+### Positive
+
+-   Significantly increases the cost of selfish mining.
+-   Reduces the ability of a single algorithm to reorg multiple blocks.
+-   Preserves the multi-algorithm decentralization objective.
+
+### Negative
+
+-   Requires a hard fork.
+-   Changes block-time dynamics under certain hash rate distributions.
+-   May introduce more short-term variance in block intervals.
+
+### Neutral
+
+-   Does not change the geometric-mean comparison logic directly.
+-   Does not modify the LWMA formula itself --- only its target-time
+    input.
+
+## References
+
+-   https://github.com/zawy12/difficulty-algorithms/issues/3#issuecomment-442129791
+-   https://github.com/zcash/zcash/issues/4021
+
+
+## Change History
+
+### 2026-06-11
+
+* Document Created.

src/SUMMARY.md

@@ -7,6 +7,7 @@
   - [TIP-PROC-3: Community Forums](TIP-0003_community_forums.md)
   - [TIP-RFC-O-8003: Tari Use Cases](RFC-8003_TariUseCases.md)
 - [Accepted](accepted.md)
+  - [TIP-RFC-MT-0004: MinoTari PoW difficulty changes](RFC/MinoTari/TIP-RFC-MT-0004_MinoTari_PoW_Difficulty_changes.md)
   - [TIP-RFC-MT-0241: XMR Atomic swap](RFC-0241_AtomicSwapXMR.md)
 - [Implemented](implemented.md)
   - [TIP-PROC-2: The Tari Community Charter](TIP-0002_tari_community_charter.md)