feat: accept fractional .freq ndec, plus compat sweep edge cases

CHANGELOG.md

@@ -19,6 +19,14 @@ breaking changes to the API or the command-line interface; patch releases
   with cells `(w/2)·scale_factor^min(|tan θ|, |cot θ|)`, and a
   line-numbered warning that this approximates FastHenry's staircase
   refinement. Native mode still rejects it.
+- Deck reader: a fractional `.freq ndec` (issue #154), as the User's Guide
+  documents, in every mode: any `ndec > 0` samples `fmin · 10^(k/ndec)`, so
+  `fmin=1e3 fmax=1e7 ndec=0.5` solves 1e3, 1e5 and 1e7 Hz. Previously any
+  non-integer `ndec` was rejected. `--freq` accepts a fractional `NDEC` too.
+  A negative `ndec` stays a line-numbered error everywhere. Under
+  `--fasthenry-compat`, a point is kept while it is ≤ 1.001 · `fmax`, and
+  `ndec=0` (read as 0.01), an omitted `ndec` (read as 1) and `fmin > fmax`
+  (an empty sweep) are line-numbered warnings instead of errors.
 - Deck reader: `relx=` / `rely=` / `relz=` on a corner-point `G` statement
   (issue #146), the documented offset (User's Guide §1.3.9), in every mode.
   It is added to every node-reference, hole and contact coordinate on the

docs/fasthenry-compat.md

@@ -82,8 +82,14 @@ Status key:
 
 | Field | Status | Notes |
 |---|---|---|
-| `.freq fmin= fmax= ndec=` | Supported | Log-spaced decade sweep, `ndec` points per decade; see `frequency_sweep` and the module's `# Semantics` section for the exact formula. |
-| `fmin = fmax` | Supported | The single-frequency case, at any `ndec`. |
+| `.freq fmin= fmax= ndec=` | Supported | Log-spaced decade sweep, `ndec` points per decade: `fmin · 10^(k/ndec)` for `k = 0, 1, …` while `k/ndec ≤ log10(fmax/fmin)`; see `frequency_sweep` and the module's `# Semantics` section. The last point is the stepped value, never clamped to `fmax`; where `k/ndec` is a whole number the point is exactly `fmin · 10^q`. `--freq` uses the same rule. |
+| Fractional `ndec` (`ndec=0.5`, `ndec=2.5`) | Supported | Issue #154. The User's Guide says `ndec` need not be an integer, and every mode accepts any `ndec > 0`: `fmin=1e3 fmax=1e7 ndec=0.5` solves 1e3, 1e5, 1e7 Hz; `ndec=0.3` solves 1e3 and 2.15443e6 Hz; `ndec=0.2` solves 1e3 Hz alone. `inp.rs`'s `fractional_ndec_reproduces_the_measured_table` pins the measured table in both modes. |
+| `ndec < 0` | Error | A line-numbered error in every mode (FastHenry never terminates on one). |
+| `ndec = 0` | Error, warns under `--fasthenry-compat` | Native: a line-numbered error. Compat: reads as `ndec=0.01`, as FastHenry does, with a warning on the `.freq` line. |
+| `ndec` omitted | Error, warns under `--fasthenry-compat` | Native: a line-numbered error. Compat: reads as `ndec=1`, with a warning on the `.freq` line. |
+| Points just above `fmax` | Supported, differs | Native keeps a point only when it is ≤ `fmax` (up to rounding). FastHenry keeps it while ≤ 1.001 · `fmax`, so `fmin=1 fmax=9.995 ndec=1` solves 1 and 10 Hz there and 1 Hz here; `--fasthenry-compat` uses FastHenry's 0.1% slack. |
+| `fmin > fmax` | Error, warns under `--fasthenry-compat` | Native: a line-numbered error. Compat: FastHenry's empty sweep (or `fmin` alone when it is within the 0.1% slack of `fmax`), with a warning on the `.freq` line; an empty sweep leaves nothing to solve unless `--freq` overrides it. |
+| `fmin = fmax` | Supported | The single-frequency case, at any `ndec > 0`. |
 | `fmin = 0` | Supported, differs | Allowed only when `fmin = fmax` (the DC solve) — `log10(0)` is undefined, so a genuine log sweep starting at zero has no well-defined first point. Rejected with a line-numbered error rather than silently starting somewhere else. |
 
 ## `.equiv`

fasterhenry-cli/README.md

@@ -42,7 +42,7 @@ Arguments:
 
 Options:
       --freq <FMIN_HZ> <FMAX_HZ> <NDEC>
-          Override the deck's sweep: min Hz, max Hz, points per decade (decade-sampled, log-spaced; FMIN == FMAX runs one frequency)
+          Override the deck's sweep: min Hz, max Hz, points per decade (decade-sampled, log-spaced; NDEC may be fractional but must be > 0; FMIN == FMAX runs one frequency)
 
       --json <OUT_JSON>
           Write the JSON result to this file instead of stdout

fasterhenry-cli/src/cli.rs

@@ -81,7 +81,8 @@ pub struct RunArgs {
     /// extension).
     pub input: PathBuf,
     /// Override the deck's sweep: min Hz, max Hz, points per decade
-    /// (decade-sampled, log-spaced; FMIN == FMAX runs one frequency).
+    /// (decade-sampled, log-spaced; NDEC may be fractional but must be > 0;
+    /// FMIN == FMAX runs one frequency).
     #[arg(
         long,
         value_names = ["FMIN_HZ", "FMAX_HZ", "NDEC"],