set up barebones Convergence Struct and print

src/convergence.jl

@@ -1,2 +1,73 @@
 
+export @logconvergence
 
+@enum Direction::UInt8 Descending Ascending
+
+const FN = Union{Float64,Nothing}
+const DN = Union{Direction,Nothing}
+
+struct Convergence
+    id::UUID
+    parentid::UUID
+    names::Vector{String}
+    values::Vector{FN}
+    thresholds::Vector{FN}
+    directions::Vector{DN}
+    done::Bool
+    step::Union{Int,Nothing}
+    function Convergence(id, parentid, names, values, thresholds, directions, done, step)
+        values = convert(Vector{FN}, values)
+        thresholds = convert(Vector{FN}, thresholds)
+        @. values[isnan_safe(values)] = nothing
+        @. thresholds[isnan_safe(thresholds)] = nothing
+        return new(id, parentid, names, values, thresholds, directions, done, step)
+    end
+end
+
+isnan_safe(v::Real) = isnan(v)
+isnan_safe(::Nothing) = false
+
+Convergence(;
+        id::UUID,
+        parentid::UUID = ROOTID,  # not nested by default
+        values=FN[nothing],
+        names=fill("", length(values)),
+        thresholds=Vector{FN}(nothing, length(values)),
+        directions=Vector{DN}(nothing, length(values)),
+        done::Bool = false,
+        step::Union{Int,Nothing} = nothing
+    ) = Convergence(id, parentid, names, values, thresholds, directions, done, step)
+
+Convergence(id::UUID; kwargs...) = Convergence(; id = id, kwargs...)
+
+# Define `string`/`print` so that progress log records are (somewhat)
+# readable even without specific log monitors.
+function Base.print(io::IO, c::Convergence)
+    for args in zip(c.names, c.values, c.thresholds, c.directions)
+        c.step !== nothing && print(io, "$(c.step) ")
+        _print_convergence(io, c.parentid, args...)
+        println(io)
+    end
+    return nothing
+end
+
+dir_char(d::Direction) = d === Descending ? '\u2193' : '\u2191' # ↓ : ↑
+dir_char(::Nothing) = '|'
+
+# print a single minutia of the convergence struct
+function _print_convergence(io, parentid, name, value, threshold, direction)
+    print(io, isempty(name) ? "Convergence" : name)
+    if parentid !== ROOTID
+        print(io, " (sub)")
+    end
+    print(io, ": ")
+    value === nothing ? print(io, "??") : print(io, value)
+    print(io, dir_char(direction))
+    threshold === nothing ? print(io, "??") : print(io, threshold)
+
+    return nothing
+end
+
+macro logconvergence()
+    :(nothing)
+end