Various improvements and changed encoding/decoding format to LLL-NNN-LLC with reduced character set.

README.md

@@ -10,36 +10,36 @@ high-integrity identifiers. It maps a compact 32-bit integer space to a
 human-readable serial format:
 
 ```text
-LLL-NNN-LC
+LLL-NNN-LLC
 ```
 
 Where:
-- `L` = data letter (`A`–`Z`)
+- `L` = data letter (`A` – `Z` excluding `F`, `I`, `O`, `Q`, `U`)
 - `N` = number (`100`–`999`)
-- `C` = checksum letter (`A`–`Z`)
+- `C` = checksum letter (`A` – `Z`)
 
 For instance 
 
 ```text
-ABC-123-DT
+ABC-123-DEO
 ```
 
 ## What it provides
 
-- 4 data letters
+- 5 data letters
 - 3 numeric digits (`100`–`999`)
 - 1 checksum letter
-- Optional forbidden-triplet filtering on the first three letters
+- Optional forbidden-triplet letter combination filtering on the first three letters
 - Reversible integer encoding/decoding
 - Deterministic checksum generation
 
-Total index capacity: **386,942,400** (`26^4 * 900`)
+Total index capacity: **3,675,690,900** (`21^5 * 900`)
 
 ## Features
 
-- Reversible mapping: `Encode(code) ↔ Decode(index)`
+- Reversible mapping: `Decode(code) ↔ Encode(index)`
 - Deterministic checksum: `C = 'A' + (index % 26)`
-- Compact index space: fits in `uint32`
+- Compact index space: fits in `uint`
 - Human-readable serial format
 - Useful for manufacturing, logistics, SaaS identifiers, and audit-safe systems
 
@@ -55,14 +55,14 @@ import (
 )
 
 func main() {
-	idx := uint32(123456)
+	idx := uint(123456)
 
-	code, err := serial.Decode(idx)
+	code, err := serial.Encode(idx)
 	if err != nil {
 		panic(err)
 	}
 
-	idx2, err := serial.Encode(code)
+	idx2, err := serial.Decode(code)
 	if err != nil {
 		panic(err)
 	}
@@ -74,21 +74,21 @@ func main() {
 
 ## Notes
 
-- The current serial format is `LLL-NNN-LC`.
+- The current serial format is `LLL-NNN-LLC`.
 - Forbidden-triplet logic is centralized in `serial.IsForbiddenTriplet` and can be tailored to project rules.
 
 ## API
 
-- `serial.Encode(code string) (uint32, error)`
-	- Parses and validates `LLL-NNN-LC`, checks checksum, and returns the deterministic index.
-- `serial.Decode(idx uint32) (string, error)`
-	- Converts a valid index back to `LLL-NNN-LC`.
-- `serial.CompleteCode(codeWithoutChecksum string) (string, uint32, error)`
-	- Takes `LLL-NNN-L` and returns the completed `LLL-NNN-LC` plus index.
-- `serial.RandomCode(isInUse ...func(uint32) bool) (string, uint32, error)`
-	- Generates a random valid `LLL-NNN-LC` code and its corresponding index.
+- `serial.Decode(code string) (uint, error)`
+	- Parses and validates `LLL-NNN-LLC`, checks checksum, and returns the deterministic index.
+- `serial.Encode(idx uint) (string, error)`
+	- Converts a valid index back to `LLL-NNN-LLC`.
+- `serial.CompleteCode(codeWithoutChecksum string) (string, uint, error)`
+	- Takes `LLL-NNN-L` and returns the completed `LLL-NNN-LLC` plus index.
+- `serial.RandomCode(isInUse ...func(uint) bool) (string, uint, error)`
+	- Generates a random valid `LLL-NNN-LLC` code and its corresponding index.
 	- If provided, the callback is used to skip indices already in use by the client.
-- `serial.RandomCodeWithOptions(options serial.RandomCodeOptions) (string, uint32, error)`
+- `serial.RandomCodeWithOptions(options serial.RandomCodeOptions) (string, uint, error)`
 	- Configurable variant with `MaxAttempts`, `IsInUse`, and custom `RandomIndex` source.
 
 Example with options:
@@ -117,7 +117,7 @@ Exported errors:
 
 - `serial.ErrInvalidFormat`
 - `serial.ErrInvalidFormatNoChecksum`
-- `serial.ErrInvalidLetters`
+- `serial.ErrInvalidLetter`
 - `serial.ErrForbiddenLetterTriplet`
 - `serial.ErrInvalidNumber`
 - `serial.ErrNumberOutOfRange`