Closes #10

README.md

@@ -381,6 +381,7 @@ Fields:
 - `rawValue`
 - `physicalValue`
 - `valid`
+- `status`
 
 ---
 
@@ -444,7 +445,7 @@ Main runtime decoder class.
 Responsibilities:
 
 - find a frame definition by CAN ID
-- decode all signals in a frame
+- decode unconditional signals and the active multiplexed branch
 - extract raw values
 - sign-extend signed values
 - convert raw values into physical values
@@ -465,6 +466,7 @@ The current parser supports the following DBC elements:
 - `CM_ BO_`
 - `CM_ SG_`
 - `VAL_` signal value descriptions
+- basic `M` / `mN` multiplexing
 
 ---
 
@@ -560,15 +562,64 @@ and scientific notation. The entire numeric token must be valid; hexadecimal
 numbers, NaN, infinity, conversion overflow, and conversion underflow are
 rejected. Minimum must not exceed maximum. Zero and negative factors are allowed.
 
-Multiplexing markers (`M`, `mN`, and combinations) are currently rejected rather
-than interpreted as part of a signal name. Multiplexing support and duplicate
-frame/signal detection remain separate work.
+Basic multiplexing markers `M` and `mN` are supported as described below.
+Overlapping signals are rejected unless their branch values differ.
+Duplicate frame/signal name detection remains separate work.
 
 Invalid signal definitions stop parsing with `path:line: description`, including
 when the signal exceeds the declared payload length.
 
 ---
 
+## Basic multiplexing
+
+```dbc
+BO_ 256 Status: 3 ECU
+ SG_ Mode M : 0|8@1+ (1,0) [0|255] "" ECU
+ SG_ Temperature m1 : 8|8@1- (1,0) [-128|127] "C" ECU
+ SG_ Pressure m2 : 8|8@1+ (2,0) [0|510] "kPa" ECU
+ SG_ Counter : 16|8@1+ (1,0) [0|255] "" ECU
+```
+
+`SignalInfo` and `DecodeSignal` contain `multiplexing`: a `MultiplexRole`
+(`Unconditional`, `Selector`, or `Branch`) and an exact `uint64_t` branch `value`.
+Each frame may have one unsigned selector (`M`); a branch (`mN`) references that
+selector implicitly. The selector may appear after its branches. `N` must contain
+only decimal digits, fit `uint64_t`, and fit the selector's bit width. Leading
+zeros are accepted. A selector without branches is allowed. Signed selectors,
+missing selectors, multiple selectors, nested/combined markers, and extended
+`SG_MUL_VAL_` statements are rejected with the originating `path:line`.
+The dispatcher registers the unsupported statement handler through `TypeFactory`.
+
+Signal bits may overlap only between branches with different selector values.
+This validation uses Intel or Motorola bit traversal, including mixed byte orders.
+It also rejects overlapping unconditional signals. Metadata survives the runtime
+and tree builders; the optional Qt adapter does not add multiplexing roles yet.
+
+The shared decoder extracts the selector before processing branches, compares its
+**raw unsigned value**, and leaves result order equal to definition order. Factor,
+offset, and `VAL_` descriptions do not influence branch selection. Unconditional
+signals continue to decode. An unmapped selector value leaves all branches inactive.
+
+Each result has `SignalDecodeStatus status`:
+
+| Status | Meaning |
+| --- | --- |
+| `Valid` | Raw and physical values are available; `valid == true` |
+| `Inactive` | Selector is available and chooses another branch |
+| `SelectorUnavailable` | Branch activation is unknown because selector bytes are missing |
+| `InsufficientPayload` | Selected or unconditional signal bytes are missing |
+| `InvalidDefinition` | Invalid bit length or missing/invalid selector in manually built runtime metadata |
+
+All statuses except `Valid` have `valid == false`; their numeric fields contain
+no decoded data and `GetValueDescription()` returns nullptr. Check `status` to
+distinguish inactive branches from errors. Consumers constructing runtime metadata
+manually must obey the same non-overlap rules as the parser. Extended and nested
+multiplexing are outside this stage and are explicitly rejected, rather than
+silently decoded as unconditional signals.
+
+---
+
 ## Comments
 
 Frame comment example:
@@ -631,7 +682,8 @@ RawCanFrame
    ↓
 Find frame in DecodeDatabase
    ↓
-For each signal:
+Extract multiplexor selector first (when present)
+For each unconditional or active signal:
     extract raw bits
     apply sign extension if needed
     convert to physical value
@@ -895,7 +947,7 @@ This is still a minimal implementation.
 
 Not supported yet:
 
-- multiplexed signals
+- extended and nested multiplexing (`SG_MUL_VAL_`, `mNM`)
 - named `VAL_TABLE_` tables and environment-variable value descriptions
 - `BA_` attributes
 - `BA_DEF_` definitions
@@ -949,7 +1001,7 @@ Recommended additions:
 
 Recommended additions:
 
-- multiplexing support
+- basic multiplexing (implemented)
 - attribute parsing
 - richer J1939 support
 - CSV export of decoded traces