add description of arbitrary initial height (#135)

spec/abci/abci.md

@@ -239,6 +239,7 @@ via light client.
   - `ConsensusParams (ConsensusParams)`: Initial consensus-critical parameters.
   - `Validators ([]ValidatorUpdate)`: Initial genesis validators, sorted by voting power.
   - `AppStateBytes ([]byte)`: Serialized initial application state. Amino-encoded JSON bytes.
+  - `InitialHeight (int64)`: Height of the initial block (typically `1`).
 - **Response**:
   - `ConsensusParams (ConsensusParams)`: Initial
     consensus-critical parameters (optional).
@@ -498,9 +499,8 @@ via light client.
   - `ChainID (string)`: ID of the blockchain
   - `Height (int64)`: Height of the block in the chain
   - `Time (google.protobuf.Timestamp)`: Time of the previous block.
-    For heights > 1, it's the weighted median of the timestamps of the valid
-    votes in the block.LastCommit.
-    For height == 1, it's genesis time.
+    For most blocks it's the weighted median of the timestamps of the valid votes in the
+    block.LastCommit, except for the initial height where it's the genesis time.
   - `LastBlockID (BlockID)`: Hash of the previous (parent) block
   - `LastCommitHash ([]byte)`: Hash of the previous block's commit
   - `ValidatorsHash ([]byte)`: Hash of the validator set for this block

spec/core/data_structures.md

@@ -349,10 +349,11 @@ ChainID must be less than 50 bytes.
 
 ```go
 block.Header.Height > 0
+block.Header.Height >= state.InitialHeight
 block.Header.Height == prevBlock.Header.Height + 1
 ```
 
-The height is an incrementing integer. The first block has `block.Header.Height == 1`.
+The height is an incrementing integer. The first block has `block.Header.Height == state.InitialHeight`, derived from the genesis file.
 
 ### Time
 
@@ -371,7 +372,7 @@ The timestamp of the first block must be equal to the genesis time (since
 there's no votes to compute the median).
 
 ```
-if block.Header.Height == 1 {
+if block.Header.Height == state.InitialHeight {
     block.Header.Timestamp == genesisTime
 }
 ```
@@ -494,7 +495,7 @@ Arbitrary length array of arbitrary length byte-arrays.
 The first height is an exception - it requires the `LastCommit` to be empty:
 
 ```go
-if block.Header.Height == 1 {
+if block.Header.Height == state.InitialHeight {
   len(b.LastCommit) == 0
 }
 ```
@@ -563,7 +564,7 @@ Once a block is validated, it can be executed against the state.
 The state follows this recursive equation:
 
 ```go
-state(1) = InitialState
+state(initialHeight) = InitialState
 state(h+1) <- Execute(state(h), ABCIApp, block(h))
 ```
 
@@ -579,8 +580,11 @@ func Execute(s State, app ABCIApp, block Block) State {
 
 	nextConsensusParams := UpdateConsensusParams(state.ConsensusParams, ConsensusParamChanges)
 	return State{
+		ChainID:         state.ChainID,
+		InitialHeight:   state.InitialHeight,
 		LastResults:     abciResponses.DeliverTxResults,
 		AppHash:         AppHash,
+		InitialHeight:   state.InitialHeight,
 		LastValidators:  state.Validators,
 		Validators:      state.NextValidators,
 		NextValidators:  UpdateValidators(state.NextValidators, ValidatorChanges),

spec/core/state.md

@@ -15,18 +15,24 @@ validation.
 
 ```go
 type State struct {
+    ChainID        string
+    InitialHeight  int64
+
     Version     Version
     LastResults []Result
-    AppHash []byte
+    AppHash     []byte
 
     LastValidators []Validator
-    Validators []Validator
+    Validators     []Validator
     NextValidators []Validator
 
     ConsensusParams ConsensusParams
 }
 ```
 
+The chain ID and initial height are taken from the genesis file, and not changed again. The
+initial height will be `1` in the typical case, `0` is an invalid value.
+
 Note there is a hard-coded limit of 10000 validators. This is inherited from the
 limit on the number of votes in a commit.